Docs: comparison with mysql++, libmysqlclient, sqlpp11

Add a comment to state the differences with these two libraries.

From the Boost review:

* Add a performance page in the documentation comparing the library with these two.
* Add a comparison to other C asynchronous APIs, in terms of ease of use.
* Clarify that the network protocol we're using here is stable.

Start by locating the existing documentation and the Boost review context; no file or test path is named in the issue. Add the requested comparisons with mysql++, libmysqlclient, sqlpp11, and other C asynchronous APIs, plus a clarification about protocol stability, and confirm that each review request is covered.

Written by the indexing model from the issue text.