DOCS: Improves DOCF-5483.

[benoitalain]

Description

Adds a link in Input System's "UI Support" documentation to the UI Toolkit manual page that contains more explanations on input handling. This should end up effectively addressing the problem of lack of clarity and scarcity of information mentioned in the DOCF-5483 jira task.

Testing status & QA

No testing. This is documentation only. That being said, I'll defer to Ben Pitt to verify that the documentation changes are showing properly.

Overall Product Risks

• Complexity: minimal
• Halo Effect: minimal

Comments to reviewers

Documentation only. No new entry, just a small improvement on an existing manual page. Do I need to add an entry to the changelog?

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[benoitalain]

@duckets I landed another PR on trunk for the UI Toolkit side of this documentation: https://github.cds.internal.unity3d.com/unity/unity/pull/58531

However, that will only show up in the 6000.1 documentation (and it doesn't show up yet as of today even though it landed a few hours ago). I guess until trunk's documentation 1) is updated and 2) points to 6000.1 by default, the DOCF-5483 task can't be fully completed. But those two things should happen by themselves without anything more from us, so I'll try to land this either way,