Update Server Selection options in connection string docs

Driver specs are introducing new options regularly. I noticed that serverSelectionTryOnce is not documented. See the Server Selection Spec for more.

I suggest the docs team review all the published specs and make sure the connection string documentation is up to date.

For example, here is a list of options from the C driver that could be used as a starting point: http://mongoc.org/libmongoc/current/mongoc_uri_t.html