Define behavior of changes to initialState value

[rubencodes]

This PR adds information about what happens if the initialState argument to useState changes value on subsequent re-renders (which is: nothing, it's ignored).

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need the corporate CLA signed.

If you have received this in error or have any questions, please contact us at cla@fb.com. Thanks!

[reactjs-bot]

Deploy preview for reactjs ready!

Built with commit 97e5ce4

https://deploy-preview-1754--reactjs.netlify.com

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[BartoszKlonowski]

Hello @rubencodes,
this is a valid note, thank you for the contribution!
Looking at the latest docs describing the useState hook I see that the fact of ignoring the further changes to initial state is already mentioned in Avoiding recreating the initial state section.
Let me close this PR as outdated, however please feel encouraged to create new PRs for open issues.