Update "Debugging Slow Builds" docs

This updates the debugging slow builds docs to give more details about
what NINJA_SUMMARIZE_BUILD=1 does, and how to interpret the weighted
time numbers.

It also adds a mention of tools/buildstate.py, and updates the
instructions for finding build information on the bots.

Bug: None
Change-Id: I3eb6900012a577569c0e123e176221bf8889b768
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4766017
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1181532}

build/docs/debugging_slow_builds.md

@@ -10,8 +10,19 @@ cd out/Default
 post_build_ninja_summary.py
 ```
 
+Because the build is highly parallelized the `elapsed time` values are usually
+not meaningful so the `weighted time` numbers are calculated to approximate
+the impact of build steps on wall-clock time.
+
 You can also set `NINJA_SUMMARIZE_BUILD=1` to have this command run
-after each `autoninja` invocation (also runs ninja with `-d stats`).
+after each `autoninja` invocation. Setting this environment variable also runs
+ninja with `-d stats` which causes it to print out internal information such
+as StartEdge times, which measures the times to create processes, and it
+modifies the `NINJA_STATUS` environment variable to add information such as how
+many processes are running at any given time - both are useful for detecting
+slow process creation. You can get this last benefit on its own by setting
+`NINJA_STATUS=[%r processes, %f/%t @ %o/s : %es ] ` (trailing space is
+intentional).
 
 To generate a Chrome trace of your most recent build:
 
@@ -21,11 +32,14 @@ ninjatracing/ninjatracing out/Default/.ninja_log > trace.json
 # Then open in https://ui.perfetto.dev/
 ```
 
+If your build is stuck on a long-running build step you can see what it is by
+running `tools/buildstate.py`.
+
 ## Slow Bot Builds
 
 Our bots run `ninjatracing` and `post_build_ninja_summary.py` as well.
 
-Find the trace at: `postprocess_for_goma > upload_log > ninja_log`:
+Find the trace at: `postprocess for reclient > gsutil upload ninja_log > ninja_log`:
 
  * _".ninja_log in table format (full)"_ is for `post_build_ninja_summary.py`.
  * _"trace viewer (sort_by_end)"_ is for `ninjatracing`.