MdeModulePkg SmmMemoryAttribute.h: Refine some comments

MdeModulePkg SmmMemoryAttribute.h: Refine some comments

1. Fix some "support" to "supported".
2. Fix some "set" to "clear" in ClearMemoryAttributes interface.
3. Remove redundant comments for GetMemoryAttributes interface.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>