TRANSLATORS comments are not being extracted

[Rafael Fontenelle]

Hi all.

I was about to suggest some "translators" comment to be added, when I
realized that it seems their are not being extracted from texi files
when generating .pot files.

In theory, po/doc/local.mk defines target "doc-pot-update' that runs
po4a-update which extract strings from the guix.texi and
contributing.texi files and store them in guix-manual.pot, which will
update all .po files in other make-target.

I notice at least one occurrence: "doc/guix.texi" in line 117 has a
comment that is clearly to be read by translators in the following.
See lines below:
-----------------------------
@c TRANSLATORS: You can replace the following paragraph with information on
@c how to join your own translation team and how to report issues with the
@c translation.
This manual is also available in Simplified Chinese (@pxref{Top,,, guix.zh_CN,
GNU Guix参考手册}), French (@pxref{Top,,, guix.fr, Manuel de référence de GNU
Guix}), German (@pxref{Top,,, guix.de, Referenzhandbuch zu GNU Guix}),
Spanish (@pxref{Top,,, guix.es, Manual de referencia de GNU Guix}), and
Russian (@pxref{Top,,, guix.ru, Руководство GNU Guix}).  If you
would like to translate it in your native language, consider joining the
@uref{https://translationproject.org/domain/guix-manual.html, Translation
Project}.
-----------------------------

The guix-manual.pot is generated with the following string (please
notice the lack of comment):
-----------------------------
#. type: Plain text
#: doc/guix.texi:128
msgid ""
"This manual is also available in Simplified Chinese (@pxref{Top,,, guix."
"zh_CN, GNU Guix参考手册}), French (@pxref{Top,,, guix.fr, Manuel de "
"référence de GNU Guix}), German (@pxref{Top,,, guix.de, Referenzhandbuch zu "
"GNU Guix}), Spanish (@pxref{Top,,, guix.es, Manual de referencia de GNU "
"Guix}), and Russian (@pxref{Top,,, guix.ru, Руководство GNU Guix}).  If you "
"would like to translate it in your native language, consider joining the "
"@uref{https://translationproject.org/domain/guix-manual.html, Translation "
"Project}."
msgstr ""
-----------------------------

Any suggestion?