Clarifies the function for the upcoming command graph linearization.

Bug: angleproject:4029
Change-Id: Ib50997c007053757de533f69c155a92d7555e0b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2003235
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>