maphash: improve docstring

[Paul Pogonyshev]

Docstring for `maphash' doesn't give any answer to the following
question: is the callback FUNCTION allowed to access the hash
table in read mode (I guess yes) and write mode?  About the
latter I have no idea without studying internal implementation.

E.g., is the following code going to work as a way to filter a
hash table?

    (maphash (lambda (key value)
               (unless (i-like-it value)
                 (remhash key hash-table)))
             hash-table)

Paul