MdePkg: Make the FileHandleGetFileName return the file name in right format.

MdePkg: Make the FileHandleGetFileName return the file name in right format.

1. If the file is not a directory remove the redundant '\' char in file name string returned from UefiFileHandleLib.FileHandleGetFileName.
2. Update function comments.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18314 6f19259b-4bc3-4df7-8a09-765794883524