Update the help information of GenBootSector according to EDK II Tools Design Spec

Signed-off-by: Yuxin Zheng <yuxin.zheng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://edk2-buildtools.svn.sourceforge.net/svnroot/edk2-buildtools/trunk/BaseTools@2574 c2973edb-eda0-4c78-bc6a-9341b269661f
Update the help information of GenPage  according to EDK II Tools Design Spec

Signed-off-by: Yuxin Zheng <yuxin.zheng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://edk2-buildtools.svn.sourceforge.net/svnroot/edk2-buildtools/trunk/BaseTools@2575 c2973edb-eda0-4c78-bc6a-9341b269661f