CmdPal: Add hints about bookmark placeholders to the Add/Edit Bookmark form (#42793)

## Summary of the Pull Request

This PR adds a short explanation to the Add/Edit Bookmark form,
describing how to use placeholders in bookmark URLs or paths

<img width="823" height="525" alt="image"
src="https://github.com/user-attachments/assets/b66adfc4-2dbc-4934-8796-9d4ad46d9f5f"
/>

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #42265 
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Bookmark/Pages/AddBookmarkForm.cs

@@ -30,25 +30,72 @@ internal sealed partial class AddBookmarkForm : FormContent
             "type": "Input.Text",
             "style": "text",
             "id": "bookmark",
-            "value": {{JsonSerializer.Serialize(url, BookmarkSerializationContext.Default.String)}},
-            "label": "{{Resources.bookmarks_form_bookmark_label}}",
+            "value": {{EncodeString(url)}},
+            "label": {{EncodeString(Resources.bookmarks_form_bookmark_label)}},
             "isRequired": true,
-            "errorMessage": "{{Resources.bookmarks_form_bookmark_required}}"
+            "errorMessage": {{EncodeString(Resources.bookmarks_form_bookmark_required)}},
+            "placeholder": {{EncodeString(Resources.bookmarks_form_bookmark_placeholder)}}
         },
         {
             "type": "Input.Text",
             "style": "text",
             "id": "name",
-            "label": "{{Resources.bookmarks_form_name_label}}",
-            "value": {{JsonSerializer.Serialize(name, BookmarkSerializationContext.Default.String)}},
-            "isRequired": false,
-            "errorMessage": "{{Resources.bookmarks_form_name_required}}"
+            "label": {{EncodeString(Resources.bookmarks_form_name_label)}},
+            "value": {{EncodeString(name)}},
+            "isRequired": false
+        },
+        {
+            "type": "RichTextBlock",
+            "inlines": [
+                {
+                    "type": "TextRun",
+                    "text": {{EncodeString(Resources.bookmarks_form_hint_text1)}},
+                    "isSubtle": true,
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": " ",
+                    "isSubtle": true,
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": {{EncodeString(Resources.bookmarks_form_hint_text2)}},
+                    "fontType": "Monospace",
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": " ",
+                    "isSubtle": true,
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": {{EncodeString(Resources.bookmarks_form_hint_text3)}},
+                    "isSubtle": true,
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": " ",
+                    "isSubtle": true,
+                    "size": "Small"
+                },
+                {
+                    "type": "TextRun",
+                    "text": {{EncodeString(Resources.bookmarks_form_hint_text4)}},
+                    "fontType": "Monospace",
+                    "size": "Small"
+                }
+            ]
         }
     ],
     "actions": [
         {
             "type": "Action.Submit",
-            "title": "{{Resources.bookmarks_form_save}}",
+            "title": {{EncodeString(Resources.bookmarks_form_save)}},
             "data": {
                 "name": "name",
                 "bookmark": "bookmark"
@@ -59,6 +106,8 @@ internal sealed partial class AddBookmarkForm : FormContent
 """;
     }
 
+    private static string EncodeString(string s) => JsonSerializer.Serialize(s, BookmarkSerializationContext.Default.String);
+
     public override CommandResult SubmitForm(string payload)
     {
         var formInput = JsonNode.Parse(payload);