Update the documentation for onCreateInputConnection

Update the documentation to clarify that WebView calls methods of the
InputConnection on a separate thread than the UI thread. This is,
in some sense, similar to shouldInterceptRequest. The methods
that override InputConnection methods should be careful about thread
restrictions and concurrency when calling other methods or accessing
shared members.

Test: Document update. Verified manually.
Change-Id: Ida76cd5dcc683092c05947c3044d848d0a249547