doc: Give @mdash macro an argument

This macro is used to consume whitespace so that the following sentence can
start on the next line without producing any whitespace in the output.  Its
argument is, therefore, whitespace.

This used to work in earlier versions of Texinfo, but around 6.{6,7} it
began failing because an argument was provided when it wasn't defined with
one.

RELEASES.md

@@ -20,6 +20,11 @@ Compiler
 --------
 - Input mappings will no longer emit the destination param as a dependency.
 
+Documentation
+-------------
+- `@mdash` macro now formally defines an argument, correcting errors in
+  newer versions of Texinfo (~v6.7).
+
 Linker
 ------
 - Remove exception for input map dependency processing (now that compiler no

doc/macros.texi

@@ -36,7 +36,7 @@ TAME
 
 @c insert appropriate em dash for mode
 @ifnottex
-@macro mdash
+@macro mdash{_}
 @inlinefmtifelse{html, @inlineraw{html,—}, —}
 @end macro
 @end ifnottex