doc: add table about mroonga_log_level
doc: add table about mroonga_log_level

The documentation about the value list of mroonga_log_level
is described in storage/wrapper section, but it should be
in reference manual.

[groonga-dev,01255]

Reported by Kazuhiro Isobe. Thanks!!!

mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga