Clarify when gpg2 and gpg are used in the epg-gpg-program doc string

* lisp/epg-config.el (epg-gpg-program): Doc string clarification
(bug#26090).
1 job for master in 90 minutes and 1 second (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #2717
test-all

01:30:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/auth-source-pass-tests.log
ELC lisp/auth-source-tests.elc
GEN lisp/auth-source-tests.log
ELC lisp/autoinsert-tests.elc
GEN lisp/autoinsert-tests.log
ELC lisp/autorevert-tests.elc
GEN lisp/autorevert-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds