autoninja: add comment on use_siso_default

Change-Id: Ia22ae12f15b845664e3e88f27731506f24c9b5e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6356123
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>

autoninja.py

@@ -184,6 +184,9 @@ def _siso_supported(output_dir):
     return False
 
 
+# `use_siso` value is used to determine whether siso or ninja is used,
+# and used to determine default value of `use_reclient`, so
+# this logic should match with //build/toolchain/siso.gni
 def _get_use_siso_default(output_dir):
     """Returns use_siso default value."""
     if not _siso_supported(output_dir):