DOC: config: clarify the fact that custom log format is not just for logging

DOC: config: clarify the fact that custom log format is not just for logging

The wording in the Custom log format section was still extremely centered
on logging, but it's about time to mention that these are usable for other
actions as well, otherwise it's very confusing for newcomers who try to
define a variable or header. The updated text also reminds about the risks
of safe encodings that may (rarely) mangle an output string, and encourages
to migrate away from the unquoted definition which is full of backslashes.
It would definitely deserve further improvements and refinements.