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The post_build_ninja_summary.py uses "weighted time" as one of its key
measures to summarize build performance but this is only documented in a
comment in the script itself. This adds documentation to the page that
recommends using this script for build-speed investigations (chiefly
through set NINJA_SUMMARIZE_BUILD=1).

This also updates the example output to say elapsed time instead of the
inaccurate term CPU time.

Change-Id: I148df429722a33f70123d9f3076b1b7e5e6cb347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902113Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713890}