DOC: config: document new "loggers" section and relevant uses

DOC: config: document new "loggers" section and relevant uses

loggers (plural) section may be used to defined a group of log
directives within a single section that can be used later in the
config as a replacement of "log" directive to specify log target(s),
for instance in a proxy section to inherit from existing loggers or
directly on a do-log action to send generated log to specific endpoints
only.