MdeModulePkg/.../IdeMode: actualize DRQReady*() comment blocks

MdeModulePkg/.../IdeMode: actualize DRQReady*() comment blocks

The DRQReady() and DRQReady2() functions only differ in that they poll
different status registers for BSY, ERR, and DRQ: the former looks at the
Status Register (clearing interrupt status), while the latter looks at the
Alternate Status Register (not clearing interrupt status).

They both correctly return a unique status code, EFI_NOT_READY, for the

  BSY==0 && ERR==0 && DRQ==0

case; that is, when the device reports "command complete".

However, the functions' leading comments don't explain this case, so it's
easy to miss in callers. Update the comments.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: John Snow <jsnow@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19684 6f19259b-4bc3-4df7-8a09-765794883524