Remove outdated "pure" function wording

Closes #2238

I think in the past the API was more aligned with a "pure" function but has since drifted pretty far from that.

Author: jakemac53

working/macros/feature-specification.md

@@ -1045,7 +1045,11 @@ written macro can cause. A well-behaved macro should not:
     done.
 
 *   Produce different results when run multiple times on the same code. A macro
-    should be a [pure function][] from input code to resulting code.
+    should be a idempotent, and should always generate the same declarations
+    from the same inputs.
+
+*   Use mutable global state to pass objects or information derived from one
+    phase to a macro (even itself) running in a later phase.
 
 The macro system is *not* designed to provide hard security guarantees of the
 above properties. Users should consider the macros that they apply to be trusted