a4e970a build: enable -Wdocumentation if suppressing external warnings (fanquake)
3b0078f doc: fixup -Wdocumentation issues (fanquake)
c6edcf1 build: suppress libevent warnings if supressing external warnings (fanquake)

Pull request description:

  Enable `-Wdocumentation` by taking advantage of our `--enable-suppress-external-warnings` flag. Most of the CIs are using this flag now, so any regressions should be caught.

  This also required modifying libevents flags when suppressing warnings, as depending on the version being built against, that could generate a large number of warnings. i.e:
  ```bash
  In file included from httpserver.cpp:34:
  In file included from ./support/events.h:12:
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:464:11: warning: parameter 'req' not found in the function declaration [-Wdocumentation]
     @param req a request object
            ^~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:465:11: warning: parameter 'databuf' not found in the function declaration [-Wdocumentation]
     @param databuf the data chunk to send as part of the reply.
            ^~~~~~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:467:11: warning: parameter 'call' not found in the function declaration [-Wdocumentation]
     @param call back's argument.
            ^~~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:939:4: warning: declaration is marked with '@deprecated' command but does not have a deprecation attribute [-Wdocumentation-deprecated-sync]
    @deprecated  This function is deprecated; you probably want to use
    ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:946:1: note: add a deprecation attribute to the declaration to silence this warning
  char *evhttp_decode_uri(const char *uri);
  ^
  __AVAILABILITY_INTERNAL_DEPRECATED
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:979:5: warning: declaration is marked with '@deprecated' command but does not have a deprecation attribute [-Wdocumentation-deprecated-sync]
     @deprecated This function is deprecated as of Libevent 2.0.9.  Use
     ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:987:1: note: add a deprecation attribute to the declaration to silence this warning
  int evhttp_parse_query(const char *uri, struct evkeyvalq *headers);
  ^
  __AVAILABILITY_INTERNAL_DEPRECATED
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:1002:11: warning: parameter 'query_parse' not found in the function declaration [-Wdocumentation]
     @param query_parse the query portion of the URI
            ^~~~~~~~~~~
  /usr/local/Cellar/libevent/2.1.12/include/event2/http.h:1002:11: note: did you mean 'uri'?
     @param query_parse the query portion of the URI
            ^~~~~~~~~~~
            uri
  69 warnings generated.
  ```

  Note that a lot of these have already been fixed upstream.

ACKs for top commit:
  laanwj:
    Concept and code review ACK a4e970a
  practicalswift:
    cr ACK a4e970a: automatic compiler feedback comes sooner and is more reliable than manual reviewer feedback
  jonatack:
    Light ACK a4e970a skimmed the changes, clang 11 build is clean with the change, verified -Wdocumentation build warnings with this change when a doc fix was reverted

Tree-SHA512: 57a1e30cffcc8bcceee72d85f58ebe29eae525861c70acb237541bd480c51ede89875c033042c0af376fdbb49fb7f588ef9282a47c6e78f9d4501c41f1b21eb6