WebView: clarify docs for addJavascriptInterface

Docs change only, no change in logic.

We do not support calling #addJavascriptInterface until after JavaScript
is enabled via WebSettings#setJavaScriptEnabled. Calling these methods
in the wrong order is undefined behavior (and we've seen that it's buggy
under certain conditions, e.g. if the DOM includes an <img> element).

This clarifies the point in the docs and code example.

Bug: 64899039
Test: make -j40 docs (everything looks good)
Change-Id: I8ef9eec7f038037e6b898286e4dad8a57ecad472
(cherry picked from commit aaef6827)