Documentation: Add isPostSavingLocked example to doc block

[Infinite-Null]

What?

Related to: #42125
PR: #10649

This PR adds a JSDoc @example comment to the isPostSavingLocked selector, completing the documentation for the lock and unlock post-saving API.

Why?

The post lock saving API was introduced in PR #10649, which includes the lockPostSaving and unlockPostSaving functions. Both of these functions have comprehensive JSDoc documentation with @example blocks, but the related isPostSavingLocked selector was missing its example documentation.

How?

Added a JSDoc @example block to the isPostSavingLocked selector that demonstrates:

• How to use the selector with useSelect hook in a React component
• An example showing conditional rendering based on the selector's return value

[github-actions]

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Co-authored-by: Infinite-Null <ankitkumarshah@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

Good example 👍