Update SmiManager() comments and implementation to follow the new rule clarified...

Update SmiManager() comments and implementation to follow the new rule clarified by PI 1.2.1 errata A.
1. If at least one of the handlers returns EFI_WARN_INTERRUPT_SOURCE_QUIESCED or EFI_SUCCESS then the function will return EFI_SUCCESS.
   If a handler returns EFI_SUCCESS and HandlerType is not NULL then no additional handlers will be processed.
2. If a handler returns EFI_INTERRUPT_PENDING and HandlerType is not NULL then no additional handlers will be processed and EFI_INTERRUPT_PENDING will be returned.

Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14136 6f19259b-4bc3-4df7-8a09-765794883524