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"
/>

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## 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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

.github/actions/spell-check/expect.txt

@@ -750,7 +750,7 @@ INITDIALOG
 INITGUID
 INITTOLOGFONTSTRUCT
 INLINEPREFIX
-Inlines
+inlines
 INPC
 inproc
 INPUTHARDWARE