[RNMobile] Update code comment for clarity

[SiobhyB]

Addresses feedback received at #16507 (comment)

In #16507, a conditional that communicates whether xPosts are enabled on a site was updated to check whether a site was hosted on WordPress.com. The reason for this change is that the conditional's mIsXPostsCapable boolean returns true for .org sites by default. As the app only support xPosts for WordPress.com sites using a P2-based theme, an unresponsive + icon was being displayed in the editor's toolbar for .org sites.

The updated conditional, including comment, was as follows:

// If this.mIsXPostsCapable has not been set, default to allowing xPosts
boolean enableXPosts = mSite.isUsingWpComRestApi() && (mIsXPostsCapable == null || mIsXPostsCapable);

This feedback suggested that the comment could be updated for clarity, given the change. As such, this PR proposes the following change:

// this.mIsXPostsCapable may return true for non-WP.com sites, but the app only supports xPosts for P2-based
// WP.com sites so, gate with `isUsingWpComRestApi()`
// If this.mIsXPostsCapable has not been set, default to allowing xPosts.

Regression Notes

1. Potential unintended areas of impact

As this is a change to a code comment, no user-facing regressions are predicted. The only negative, unintentional impact would be developer confusion, if the comment isn't clear.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

N/A

3. What automated tests I added (or what prevented me from doing so)

N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-16547-build-123459.apk

• WordPress-pr-16547-build-123459.apk

[hypest]

LGTM!