CXX-1176 Add doxygen tag for mongocxx and bsoncxx namespaces

Backported from CXX-1043.

Author: saghm

src/bsoncxx/json.hpp

@@ -22,9 +22,17 @@
 
 #include <bsoncxx/config/prelude.hpp>
 
+///
+/// Top level namespace for MongoDB C++ BSON functionality.
+///
 namespace bsoncxx {
 BSONCXX_INLINE_NAMESPACE_BEGIN
 
+// Placing this variable between the `BSONCXX_INLINE_NAMESPACE_BEGIN` and the Doxygen documentation
+// for `to_json` suppressed the macro name from being shown as part of the return type in the
+// generated documentation pages.
+extern const bool k_silence_doxygen;
+
 ///
 /// Converts a BSON document to a JSON string.
 ///

src/mongocxx/client.hpp

@@ -27,6 +27,9 @@
 
 #include <mongocxx/config/prelude.hpp>
 
+///
+/// Top level namespace for the MongoDB C++ driver.
+///
 namespace mongocxx {
 MONGOCXX_INLINE_NAMESPACE_BEGIN