Update doc references to ninja to also mention autoninja.

Change-Id: Ib45d3f86a0b68f6d0b10847cf7e3859b640e34a3
Reviewed-on: https://chromium-review.googlesource.com/c/1292403
Commit-Queue: Dirk Pranke <[email protected]>
Reviewed-by: Bruce Dawson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#602453}
diff --git a/docs/chromeos_build_instructions.md b/docs/chromeos_build_instructions.md
index 852fc254..164f5de 100644
--- a/docs/chromeos_build_instructions.md
+++ b/docs/chromeos_build_instructions.md
@@ -42,6 +42,9 @@
     $ autoninja -C out/Default chrome
     $ out/Default/chrome
 
+(`autoninja` is a wrapper that automatically provides optimal values for the
+arguments passed to `ninja`).
+
 Some additional options you may wish to set by passing in `--args` to `gn gen`
 or running `gn args out/Default`: