doc: Expand on the first argument to subst.

Fixes #752

docs/Language.md

@@ -444,11 +444,18 @@ program state.
     in all lowercase.
 *   `subst(old, new, val)`, a function of three arguments which returns the
     input `val` with all substrings or patterns `old` replaced by `new`.  When
-    given a string for `old`, it is a direct proxy of the Go
+    given a *string* for `old`, it is a direct proxy of the Go
     [strings.ReplaceAll](https://golang.org/pkg/strings/#ReplaceAll) function.
-    When given a regular expression pattern for `old`, it uses
+
+    `subst("old", "new", $val)`
+
+    When given a *regular expression pattern* for `old`, it uses
     [regexp.ReplaceAllLiteralString](https://golang.org/pkg/regexp/#Regexp.ReplaceAllLiteralString).
 
+    `subst(/old/, "new", $val)`
+
+    Note the different quote characters in the first argument.
+
 There are type coercion functions, useful for overriding the type inference made
 by the compiler if it chooses badly. (If the choice is egregious, please file a
 bug!)