MINOR: compression: Warn for 'compression offload' in defaults sections

MINOR: compression: Warn for 'compression offload' in defaults sections

This directive is documented as being ignored if set in a defaults
section. But it is only mentionned in a small note in the configuration
manual. Thus, now, a warning is emitted. To do so, the errors handling in
parse_compression_options() function was slightly changed.

In addition, this directive is now documented apart from the other
compression directives. This way, it is clearly visible that it must not be
used in a defaults section.