The message catalog can be used to attach short help texts to log lines,
keyed by their MESSAGE_ID= fields. This is useful to help the
administrator understand the context and cause of a message, find
possible solutions and find further related documentation.

Since this is keyed off MESSAGE_ID= this will only work for native
journal messages.

The message catalog supports i18n, and is useful to augment english
language system messages with explanations in the local language.

This commit only includes short explanatory messages for a few example
message IDs, we'll add more complete documentation for the relevant
systemd messages later on.