Clarify docs for WebView multiwindow settings.

Update the documentation for WebSettings to explain the special behavior
used when multiple window support is disabled, and to clarify that
in setJavaScriptCanOpenWindowsAutomatically, "automatically" means
"without a user gesture".

Test: m offline-sdk-docs
Flag: DOCS_ONLY
Change-Id: I0752e58f9145f7bda7c33a8f43929878bd08951b