CDRIVER-2099 Remove some deprecated documentation (#1265)

* remove docs for `bson_md5_*`

* remove documentation for mongoc_matcher_t

* remove documentation for `mongoc_client_get_max_*_size`

* use `BSON_GNUC_DEPRECATED_FOR` in bson_oid_init_sequence

The documentation suggests using bson_oid_init as an alternative.

* move "Deprecated" sections above "Synopsis"

And uses consistent verbage.

* add missing `BSON_GNUC_DEPRECATED` attributes

* add `warning` and `deprecated` directives