* doc/guix.texi (Messaging Services): Add "mam" as a module example. Document
'prosodyctl check'. Replace 'prosodyctl cert request' with 'prosodyctl cert
import'. Regenerate it.
* gnu/services/messaging.scm (serialize-module-list): Remove "posix" from the
default modules list because it is now automatically loaded.
(ssl-configuration)[key, certificate]: Remove them because they are now
automatically located. Fix their docstrings.
(%default-modules-enabled): Add "carbons" and "blocklist".
(prosody-configuration)[certificates]: Set default directory from which
certificates/keys will be automatically located.
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