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.
Use a Python source file (linecache.__file__) instead of /etc/passwd.
Modify also linecache docstrings to clarify the linecache is written to cache
Python source files, not any text files.