Check external references in all info documents in the info path.
`Info-directory-list' and `Info-additional-directory-list' are
the info paths. See `info-xref-check' for how each file is
The search for "all" info files is rather permissive, since
info files don't necessarily have a ".info" extension and in
particular the Emacs manuals normally don't. If you have a
source code directory in `Info-directory-list' then a lot of
extraneous files might be read. This will be time consuming but should be harmless. (fn)