doc: use section 3 for now.

Author: Christian Hergert

doc/mongoc_api.txt

@@ -8,10 +8,7 @@ linkmongoc:mongoc_gridfs[7] +
 linkmongoc:mongoc_gridfs_file[7] +
 linkmongoc:mongoc_gridfs_file_list[7] +
 linkmongoc:mongoc_read_prefs[7] +
-linkmongoc:mongoc_socket[7] +
 linkmongoc:mongoc_ssl[7] +
 linkmongoc:mongoc_stream[7] +
-linkmongoc:mongoc_stream_file[7] +
-linkmongoc:mongoc_stream_socket[7] +
 linkmongoc:mongoc_uri[7] +
 linkmongoc:mongoc_write_concern[7] +