WebView: clarify Safe Browsing opt-in in docs

No change to logic, just docs updates.

It looks like our documentation for the Safe Browsing manifest tag was
poor. This clarifies that the manifest opt-in should go inside the
application tag.

This also updates the documentation for MetricsOptOut, since this has
the same requirement.

Bug: 69007974
Test: make docs (verify it looks better)
Change-Id: Ieb29932f1460e3aa25c486b47692b63c0b87f509