DOC: better document the config file format and escaping/quoting rules

DOC: better document the config file format and escaping/quoting rules

It's always a pain to figure how to proceed when special characters need
to be embedded inside arguments of an expression. Let's document the
configuration file format and how unquoting/unescaping works at each
level (top level and argument level) so that everyone hopefully finds
suitable reminders or examples for complex cases.

This is related to github issue #200 and addresses issues #712 and #966.