kconfig.debug: clarify CONFIG_DEBUG_INFO help text

The following patch adds some extra clarification to the CONFIG_DEBUG_INFO
Kconfig help text.  The current text is mostly a recursive definition and
doesn't really say much of anything.  When I first read this I thought it
was going to enable extra verbosity in debug messages or something, but it
is only enabling the "gcc -g" compile option in the Makefile.

Signed-off-by: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>