doc: fix mfcalc code.

* doc/bison.texinfo (Multi-function Calc): Add missing includes.
	Fix the rendering of the result: use @result and remove the
	initial tabulation in the actual code.
	Fix stylistic issues: avoid the , operator.
	Add extexi mark-up.
	* examples/extexi: Also support @smallexample.

examples/extexi

@@ -52,11 +52,11 @@ BEGIN {
     }
 }
 
-/^@example$/, /^@end example$/ {
+/^@(small)?example$/, /^@end (small)?example$/ {
   if (!file)
     next;
 
-  if ($0 ~ /^@example$/)
+  if ($0 ~ /^@(small)?example$/)
     {
       input = files_output[file] ? "\n" : "";
 
@@ -69,7 +69,7 @@ BEGIN {
       next;
     }
 
-  if ($0 ~ /^@end example$/)
+  if ($0 ~ /^@end (small)?example$/)
     {
       if (input == "")
         fatal("no contents: " file);