MdePkg: DebugLib: more cleanup for log level comments in lib class header

MdePkg: DebugLib: more cleanup for log level comments in lib class header

The DEBUG_POOL and DEBUG_PAGE clarifications were suggested by Leif
Lindholm.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>