message: build: document that aaudio was experimental
  and make git blame point to a public description
  NOTRY=true
  Cr-Commit-Position: refs/heads/master@{#32130}
diff: @@ -120,8 +120,8 @@ declare_args() {
  # Enable to use the Mozilla internal settings.
  build_with_mozilla = false
  - # Enable use of Android AAudio which requires Android SDK 26 or above and
  - # NDK r16 or above.
  + # Experimental: enable use of Android AAudio which requires Android SDK 26 or above
  + # and NDK r16 or above.
  rtc_enable_android_aaudio = false
  # Set to "func", "block", "edge" for coverage generation.
  