Consistenty use auto-generated namespace comments.

Consistenty use auto-generated namespace comments.

This patch was generated using clang-tidy:

  clang-tidy -checks="-*,google-readability-namespace-comments" -fix

Switch to the format used by clang-tidy to allow easy, automatic,
tool-based maintenance / checking of the comments.

This also found a few instances of comments that were out of sync.