Issue lib389 #77 - Refactor docstrings in rST format - part 2
Description: Improve docstring coverage for IDM modules.
Fix small typos in replica, backend and mappingTree modules.
Add Python 3 support chapter to guidelines.rst.
Fix links to the source code because of the lib389 repo merging.
https://pagure.io/lib389/issue/77
Reviewed by: mreynolds, wibrown (Thanks!)