epa.texi: Emacs and GnuPG pinentry

[Jens Schmidt <jschmidt4gnu@vodafonemail.de>]

Hi.

I currently try to extend epa.texi based on my own experience with
GnuPG and Emacs setup.  (Bug#64154 tracks that, but it does not contain
anything overly interesting so far.)

What puzzled me during my first-time setup were the different modes of
Emacs as GnuPG Pinentry.  Basically you can have for GnuPG >= 2.1:

1. Use Emacs only for GnuPG requests that are triggered by Emacs itself
    (aka loopback pinentry),

2. use Emacs for *all* GnuPG requests (also non-Emacs ones), or

3. use Emacs for all GnuPG requests with some other non-Emacs Pinentry
    as fallback.

The latter two alternatives require pinentry.el, which was present in
Emacs core only in versions [25, ..., 27] (see
https://lists.gnu.org/archive/html/emacs-devel/2018-01/msg00233.html).
Plus they are not supported on Debian.

While some users may consider alternatives 2. and 3. desirable I'm not
sure whether I should describe their setup in detail in epa.texi.  I
could just mention them and that "they are not recommended for use".  Or
whatever.

WDYT?