'main': Document the second meaning of the 'comment' style.

Author: danielshahaf

docs/highlighters/main.md

@@ -58,6 +58,7 @@ This highlighter defines the following styles:
 * `assign` - parameter assignments (`x=foo` and `x=( )`)
 * `redirection` - redirection operators (`<`, `>`, etc)
 * `comment` - comments, when `setopt INTERACTIVE_COMMENTS` is in effect (`echo # foo`)
+* `comment` - elided parameters in command position (`$x ls` when `$x` is unset or empty)
 * `named-fd` - named file descriptor (`echo foo {fd}>&2`)
 * `arg0` - a command word other than one of those enumerated above (other than a command, precommand, alias, function, or shell builtin command).
 * `default` - everything else