#11726: Make linecache docs reflect that all files are treated the same.

#11726: Make linecache docs reflect that all files are treated the same.

Being able to read non-python text files is not a purpose of linecache, but it
does work and people use it.  This changeset adjusts the language to make it
clear that Python files are not treated uniquely, but does not go so far as to
say reading non-python files is explicitly supported.