This should resolve #177 where it was determined that the best course of action would be to document the use case of manually closing mmaps.  The other suggestion, that there be a brief explanation of mmap, was already there as it turns out (though it needed a mention of the fact that memmap=True is now the default).

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.1-stable@2131 ed100bfc-0583-0410-97f2-c26b58777a21