pw_env_setup: Document environment variables
pw_env_setup: Document environment variables

Change-Id: I4a7a36f335f5868b87c7bbf3d639cd71ec114477
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/29480
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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