Added new footnote about list.sort() stability. Repaired footnote about

Added new footnote about list.sort() stability.  Repaired footnote about
using sort() with comparison functions (it made reference to the non-
existent "builtin-in function sort()").

BTW, I changed list.sort's docstring to contain the word "stable" -- the
easiest way to tell whether a particular Python version's sort *is* stable
is to look for "stable" in the docstring.  I'm not sure whether to
advertise this <wink>.