[Python-3000] abc docs

[Georg Brandl]

I've added a basic skeleton of documentation for the "abc" module, but it
would be nice if somebody proofread it and at add more from PEP 3119 if
desired.

Georg

-- 
Thus spake the Lord: Thou shalt indent with four spaces. No more, no less.
Four shall be the number of spaces thou shalt indent, and the number of thy
indenting shall be four. Eight shalt thou not indent, nor either indent thou
two, excepting that thou then proceed to four. Tabs are right out.