Doc fix for `compilation-error-regexp-alist'

[Lars Hansen <larsh@soem.dk>]

In the docstring of `compilation-error-regexp-alist', something must be
wrong with the description of the FILE element in the case where it is a
function. It says

"Or FILE can also be a function to return the filename."

But FILE seems to be passed to `compilation-error-properties', and
judging from that functions code, the description of FILE should be
changed to

"Or FILE can also be a function to return  (FILENAME) or
(RELATIVE-FILENAME . DIRNAME). In the former case, FILENAME may be
relative or absolute."

Can someone with a better knowledge of compile.el than me confirm this?