Clarify the requirements for the Destination parameter of UnicodeStrToAsciiStr.

signed-off-by:jcarsey
Reviewed-by:lgao4

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11917 6f19259b-4bc3-4df7-8a09-765794883524

MdePkg/Include/Library/BaseLib.h

@@ -636,6 +636,9 @@ StrHexToUint64 (
   to the ASCII string Destination by copying the lower 8 bits of
   each Unicode character. It returns Destination.
 
+  The caller is responsible to make sure Destination points to a buffer with size
+  equal or greater than ((StrLen (Source) + 1) * sizeof (CHAR8)) in bytes.
+
   If any Unicode characters in Source contain non-zero value in
   the upper 8 bits, then ASSERT().