read-file-name-predicate

["Drew Adams" <drew.adams@oracle.com>]

`read-file-name-predicate' is a Lisp variable. It's doc says only that it is
the current predicate used by built-in function `read-file-name-internal'.
This is the only doc about it, and it gives the impression that it is only
an internal variable for a built-in function. If that were true, then why
would it be a Lisp variable instead of a C variable?

The C code shows that `read-file-name-predicate' is in fact set to the value
of the PREDICATE arg to `read-file-name', but that is not documented
anywhere - shouldn't it be?  `read-file-name-predicate' has an external,
Lisp behavior: During completion, Lisp code can check what the predicate is
or even change the predicate. Shouldn't that be documented?

IOW, it seems like this variable is currently half way between being
internal and external. The doc suggests that it is internal, but the code
shows that you can examine it and change it from Lisp.