[DOC] Document that certain objects require manual cleanup

[leofang]

For example, users need to call Request.Free() explicitly, see the discussion around #29 (comment).

[leofang]

This needs:

1. a short example added to Tutorial
2. docstrings added to the relevant classes (ex: Request)

[leofang]

@dalcinl If IIUC non-default instances of the following 9 (*) classes need an explicit destructor call:

• Datatype
• Win
• Request
• Op
• Info
• Group
• Comm
• Errhandler
• File (* should call Close() instead of Free())

[dalcinl]

Yes. Request is a bit special. For Request instances, Wait() or Test() methods implicitly work as a destructor. Prequest (subclass of Request) instances always require a Free() call.

The case of Comm, Win, and File is also a bit special, as the Free() or Close() calls are collective. A consequence of this is that these destructors could never be let in the hand of Python's garbage collector.