update diagnostic message.

Author: csyonghe

source/slang/slang-diagnostic-defs.h

@@ -325,7 +325,7 @@ DIAGNOSTIC(30066, Error, classCanOnlyBeInitializedWithNew, "a class can only be
 DIAGNOSTIC(30067, Error, mutatingMethodOnFunctionInputParameterError, "mutating method '$0' called on `in` parameter '$1'; changes will not be visible to caller. copy the parameter into a local variable if this behavior is intended")
 DIAGNOSTIC(30068, Warning, mutatingMethodOnFunctionInputParameterWarning, "mutating method '$0' called on `in` parameter '$1'; changes will not be visible to caller. copy the parameter into a local variable if this behavior is intended")
 
-DIAGNOSTIC(30070, Error, unsizedMemberMustAppearLast, "unsized member can only appear as the last member in a composite type.")
+DIAGNOSTIC(30070, Error, unsizedMemberMustAppearLast, "member with unknown size at compile time can only appear as the last member in a composite type.")
 DIAGNOSTIC(30071, Error, varCannotBeUnsized, "cannot instantiate a variable of unsized type.")
 
 DIAGNOSTIC(30075, Error, cannotSpecializeGeneric, "cannot specialize generic '$0' with the provided arguments.")