I agree that overall, documentation is not a "career building" activity, but there are certainly some notable exceptions where the documentation is so good that people rave about it. A few that come to mind:

  * The Bash manpage [1]
  * FreeBSD docs, in general — the "base system" implementation and
    documentation go hand-in-hand. If there's missing docs for a
    built-in tool, that's treated as a bug.
  * SQLite's documentation of the SQL language syntax [2]

  [1] https://linux.die.net/man/1/bash
  [2] https://www.sqlite.org/lang_select.html