Update startMethodTracing documentation

In API 34, flags passed to startMethodTracing were extended to configure
time source. Add documentation about the new flags.

Bug: 329499422
Test: just updated documentation
Flag: EXEMPT DOCS_ONLY
(cherry picked from https://android-review.googlesource.com/q/commit:f58be28756ea8bfc8e353432a8eaebef7da2cf86)
Merged-In: I5f1a706d9c563db7b5dfc4974217746d2ddeb262
Change-Id: I5f1a706d9c563db7b5dfc4974217746d2ddeb262