Add comments to clarify how #define gets wrapped

It took me a while to fully get my head around this, and I'd like to
help out future developers.

CHANGES.current

@@ -7,6 +7,9 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-07-27: ZackerySpytz, olly
+	    #1622 Add support for the C++20 spaceship operator (<=>).
+
 2022-07-26: olly
 	    [Tcl] https://sourceforge.net/p/swig/bugs/977/
 	    Fix handling of long long on 32-bit platforms.  This fix raises

Source/CParse/parser.y

@@ -1951,6 +1951,9 @@ clear_directive : CLEAR tm_list SEMI {
 /* ------------------------------------------------------------
    %constant name = value;
    %constant type name = value;
+
+   Note: Source/Preprocessor/cpp.c injects `%constant X = Y;` for
+   each `#define X Y` so that's handled here too.
    ------------------------------------------------------------ */
 
 constant_directive :  CONSTANT identifier EQUAL definetype SEMI {

Source/Preprocessor/expr.c

@@ -10,6 +10,11 @@
  *
  * Integer arithmetic expression evaluator used to handle expressions
  * encountered during preprocessing.
+ *
+ * Note that this is used for expressions in `#if` and the like, but not
+ * for expressions in `#define` which SWIG wraps as constants - for those
+ * we inject a `%constant` directive which is handled by the parser in
+ * `Source/CParse/parser.y`.
  * ----------------------------------------------------------------------------- */
 
 #include "swig.h"