Clarify what happens if shallow is false. Also, the caching note is not really...

Clarify what happens if shallow is false. Also, the caching note is not really correct since the cache gets occasionally cleared; therefore I removed it.

Doc/library/filecmp.rst

@@ -21,11 +21,8 @@ The :mod:`filecmp` module defines the following functions:
    Compare the files named *f1* and *f2*, returning ``True`` if they seem equal,
    ``False`` otherwise.
 
-   Unless *shallow* is given and is false, files with identical :func:`os.stat`
-   signatures are taken to be equal.
-
-   Files that were compared using this function will not be compared again unless
-   their :func:`os.stat` signature changes.
+   If *shallow* is true, files with identical :func:`os.stat` signatures are
+   taken to be equal.  Otherwise, the contents of the files are compared.
 
    Note that no external programs are called from this function, giving it
    portability and efficiency.