Clarify WebViewClient.shouldOverrideUrlLoading docs

Call out more explicitly the antipattern of calling loadUrl with the
same URL and returning true, and repeat this on the deprecated version.
Simplify the wording about returning true v.s. false. Switch to the
"note" style used elsewhere on the page.

BUG:111843379
Change-Id: I36c31a8e0f4754c314b8a4d72cc497c9c3a3e242
Test: make docs