Add note for is_official_build flag

is_official_build and is_component_build are not compatible.

Change-Id: I4d1a3c788eaf07589b24da6158e84b6ffd280d45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4915133
Reviewed-by: Oleksandr Peletskyi <peletskyi@google.com>
Commit-Queue: Oleh Lamzin <lamzin@google.com>
Auto-Submit: Oleh Lamzin <lamzin@google.com>
Cr-Commit-Position: refs/heads/main@{#1205720}

docs/chromeos_build_instructions.md

@@ -82,6 +82,8 @@ or running `gn args out/Default`:
 
     # Enables many optimizations, leading to much slower compiles, links,
     # and no runtime stack traces.
+    #
+    # Note: not compatible with `is_component_build = true`.
     is_official_build = true
 
 NOTE: You may wish to replace 'Default' with something like 'Cros' if