Consider the case where 2 files are shallow compared where only the mtime differs (i.e. the mode and size is identical).

With filecmp.cmp(f1, f2, shallow=True) a deep compare would be performed behind the scenes since the guard clauses fell through. This discrepancy is either a problem in the docstring or a problem in the comparison itself.

Two options remain:
- the documentation is altered and describes that, in case only the mtime differs (i.e. mode and size are equal) a deep compare is performed
- the behaviour is restricted in that effectively only a shallow compare is performed
- a shallow compare becomes more restrictive (do not consider the mtime during the compare)

My preference is to adjust the function to safeguard the meaning of 'shallow' in this context.