Explain DIGIT_ZERO (etc.) macro naming

Author: catamorphism

Diff for: icu4c/source/i18n/messageformat2_macros.h

@@ -46,6 +46,9 @@ using namespace pluralimpl;
 #define LOWERCASE_E ((UChar32)0x0065)
 #define UPPERCASE_E ((UChar32)0x0045)
 
+// The first three macro names are prefixed with DIGIT_
+// in order to avoid substituting the macros into the
+// definition of StandardPlural::Form (see standardplural.h)
 #define DIGIT_ZERO ((UChar32)0x0030)
 #define DIGIT_ONE ((UChar32)0x0031)
 #define DIGIT_TWO ((UChar32)0x0032)