Add some missing headers and ensure each header has a short description. doc.go
gets confused at declarations that break before the first (, so avoid doing
that. Also skip a/an/deprecated: in markupFirstWord and process pipe words in
the table of contents.

Change-Id: Ia08ec5ae8e496dd617e377e154eeea74f4abf435
Reviewed-on: https://boringssl-review.googlesource.com/2839
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)