[WebView Tracing API] documentation fixes.

Some fixes to the WebView Tracing API related documentation.
In particular this patch makes sure that:
- the defaults are correct,
- the examples are correct,
- the formatting is ok,
- the text for parameters etc.. is formatted consistently,
- the documentation is spellchecked and typos corrected.

BUG: 63750258
Test: manual inspection of the generated html documentation
Change-Id: I53ac1a1d003e519739464b8d8554f3d8262c46e4