FormState::set(), get(), and has() should explain they work with storage, and crosslinked with getStorage()

Problem/Motivation

API page: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21Fo...

For the following methods:
- setStorage
- getStorage
- get
- set
- has

Change doc comments from

"Sets a value to an arbitrary property."

to a more descriptive comments, and say that it sets the form state storage.

Add @see references between all of the following methods:
- setStorage
- getStorage
- get
- set
- has

Steps to reproduce

Proposed resolution

Remaining tasks

Update patch
Respond to #34
review
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User interface changes

API changes

Data model changes

Release notes snippet