MdeModulePkg: Refine the services comments in EFI_PRINT2_PROTOCOL

MdeModulePkg: Refine the services comments in EFI_PRINT2_PROTOCOL

For the following 8 services in EFI_PRINT2_PROTOCOL:
UNICODE_BS_PRINT
UNICODE_S_PRINT
UNICODE_BS_PRINT_ASCII_FORMAT
UNICODE_S_PRINT_ASCII_FORMAT
ASCII_BS_PRINT
ASCII_S_PRINT
ASCII_BS_PRINT_UNICODE_FORMAT
ASCII_S_PRINT_UNICODE_FORMAT

They will ASSERT when:
1) The input parameter 'StartOfBuffer' is NULL if 'BufferSize' indicates
at least 1 Ascii/Unicode character can be held.
2) The input parameter 'FormatString' is NULL if 'BufferSize' indicates at
least 1 Ascii/Unicode character can be held.
3) The input parameter 'FormatString' contains more than
PcdMaximum[Ascii|Unicode]StringLength Ascii/Unicode characters.
4) The produced string contains more than
PcdMaximum[Ascii|Unicode]StringLength Ascii/Unicode characters.

This commits removes the ASSERT case 4) and add the following new ASSERT
case:
4) The input parameter 'BufferSize' is greater than
(PcdMaximumAsciiStringLength * sizeof (CHAR8)) for Ascii format string or
(PcdMaximumUnicodeStringLength * sizeof (CHAR16) + 1) for Unicode format
string.

And for those ASSERT cases, 0 will be returned by those services.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>