doc/gpgsm: explain what "policy-file" refers to.

A new user who sees "policy-file" and searches naively through the
documentation to find it again won't be able to tell what this refers
to, since "policies.txt" doesn't otherwise match the search string
"policy". This gives them a fighting chance at finding the
documentation.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>