Improve unconstify() documentation

Refer to expression instead of variable when appropriate.

Discussion: https://www.postgresql.org/message-id/08adbe4e-38f8-2c73-55f0-591392371687%402ndquadrant.com

src/include/c.h

@@ -1122,14 +1122,14 @@ typedef union PGAlignedXLogBlock
 #endif
 
 /*
- * Macro that allows to cast constness away from a variable, but doesn't
+ * Macro that allows to cast constness away from an expression, but doesn't
  * allow changing the underlying type.  Enforcement of the latter
  * currently only works for gcc like compilers.
  *
- * Please note IT IS NOT SAFE to cast constness away if the variable will ever
+ * Please note IT IS NOT SAFE to cast constness away if the result will ever
  * be modified (it would be undefined behaviour). Doing so anyway can cause
  * compiler misoptimizations or runtime crashes (modifying readonly memory).
- * It is only safe to use when the the variable will not be modified, but API
+ * It is only safe to use when the the result will not be modified, but API
  * design or language restrictions prevent you from declaring that
  * (e.g. because a function returns both const and non-const variables).
  *
@@ -1137,13 +1137,13 @@ typedef union PGAlignedXLogBlock
  * be nice, but not trivial, to improve that).
  */
 #if defined(HAVE__BUILTIN_TYPES_COMPATIBLE_P)
-#define unconstify(underlying_type, var) \
-	(StaticAssertExpr(__builtin_types_compatible_p(__typeof(var), const underlying_type), \
+#define unconstify(underlying_type, expr) \
+	(StaticAssertExpr(__builtin_types_compatible_p(__typeof(expr), const underlying_type), \
 					  "wrong cast"), \
-	 (underlying_type) (var))
+	 (underlying_type) (expr))
 #else
-#define unconstify(underlying_type, var) \
-	((underlying_type) (var))
+#define unconstify(underlying_type, expr) \
+	((underlying_type) (expr))
 #endif
 
 /* ----------------------------------------------------------------