Capture/restricted_traces: update documentation With the new context handling the trace files are named differently, and the description how to remove secondary contexts also is out of date. Bug: None Change-Id: Ia8258502ef53f9d90b742eff01f8125d816ca74e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4323658 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Gert Wollny <gert.wollny@collabora.com>