style: use midrule only, not mid-rule

The code was already using midrule only, never mid_rule.  This is
simpler to remember, and matches a similar change we made from
look-ahead to lookahead.

* NEWS, doc/bison.texi, src/reader.c, src/scan-code.h, src/scan-code.l
* tests/actions.at, tests/c++.at, tests/existing.at: here.

src/scan-code.h

@@ -85,7 +85,7 @@ typedef struct code_props {
   /** Named reference. */
   named_ref *named_ref;
 
-  /** Type, for mid-rule actions.  */
+  /** Type, for midrule actions.  */
   uniqstr type;
 } code_props;