Rollup merge of #136876 - joshtriplett:locking-might-not-be-advisory, r=Amanieu

Locking documentation updates

- Reword file lock documentation to clarify advisory vs mandatory. Remove the
  word "advisory", and make it more explicit that the lock may be advisory or
  mandatory depending on platform.

- Document that locking a file fails on Windows if the file is opened only for append

Author: matthiaskrgr