Improve docs for received_response_content_length.
Improve docs for received_response_content_length.

Clarify what it returns and when its valid. Remove the comment about
only being valid for certain protocols.

BUG=

Review URL: https://codereview.chromium.org/1292543003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343520}

CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend