document fam reply is empty for w0 (#789)

If an unacknowledged write concern is used as an option for a
mongoc_find_and_modify_with_opts, the output reply document is
always empty.

Author: kevinAlbs

src/libmongoc/doc/mongoc_collection_find_and_modify_with_opts.rst

@@ -32,6 +32,8 @@ Update and return an object.
 
 ``reply`` is always initialized, and must be freed with :symbol:`bson:bson_destroy()`.
 
+If an unacknowledged write concern is set (through :symbol:`mongoc_find_and_modify_opts_append`), the output ``reply`` is always an empty document.
+
 Errors
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