Display a help text when listing EPA keys

* lisp/epa.el (epa--list-keys, epa-list-keys): Display a help text
that explains what the letters mean.
1 job for master in 57 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #4610


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
Makefile:294: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1