This change fixes up several comments (many of which were spotted by
Kenny Root) and also changes doc.go to detect cases where comments don't
start with the correct word. (This is a common error.)

Since we have docs builders now, these errors will be found
automatically in the future.

Change-Id: I58c6dd4266bf3bd4ec748763c8762b1a67ae5ab3
Reviewed-on: https://boringssl-review.googlesource.com/6440
Reviewed-by: Adam Langley <agl@google.com>