The manpage of gpg states that the PINENTRY_USER_DATA environment variable is
passed via gpg-agent to pinentry. However this doesn't seem to be true. Setting
pinentry-program in ~/.gnupg-gpg-agent.conf to a script that dumps its
environment to a file, I don't see any PINENTRY_USER_DATA when calling gpg with
the following invocation:

  gpg --sign testfile

However, the exact same thing works with gpg2.

Looking around in the source tree for gnupg, I'm confused because it seems like
PINENTRY_USER_DATA isn't mentioned anywhere except in the doc. (It occurs in
gnupg2, however.) Unless I'm getting something wrong, PINENTRY_USER_DATA should
be removed from the gnupg documentation, or support for that variable should be
actually implemented.