pw_presubmit: Add docs for upstream checks
pw_presubmit: Add docs for upstream checks

Change-Id: I5869de37d91e2ef390e30e0cbdad42d3cd084729
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46361
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed