[native] Add note about kbInitialProps styles

Summary:
Follow up https://phab.comm.dev/D6556?id=21940#inline-43588
This style wasn't used anywhere and I couldn't find anything useful in git history.

Update (see comments below):
It turns out that it is used by react-native-keyboard-input so I added a comment

Depends on D6556

Test Plan:
N/A - just added a comment.
Flow and ESLint don't complain.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6569