Documentation and other cleanup in dart:ui, plus a small performance improvement

[Hixie]

• Remove nonsensical (and I think unnecessary?) comment.
• Document willChangeHint, isComplexHint
• Fix grammer in Rect docs
• Remove runtimeType.toString twice
• Add detail to ImageShader constructor docs.
• Document Vertices and drawVertices!
• Update checks in Vertices constructors (and add tests).
• Fix some typos.
• Minor other doc improvements.
• Fold in @jonahwilliams' performance improvement from [ui] make vertices validation slightly cheaper #38041

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.