tutorial.md: no need to mention heads(x) anymore

Instead we point explicitly at the full documentation on revsets.

Suggested-by: Martin von Zweigbergk <[email protected]>

docs/tutorial.md

@@ -161,9 +161,8 @@ function in the revset matches it.
 
 There are also operators for getting the parents (`foo-`), children (`foo+`),
 ancestors (`::foo`), descendants (`foo::`), DAG range (`foo::bar`, like
-`git log --ancestry-path`), range (`foo..bar`, same as Git's). There are also a
-few more functions, such as `heads(<set>)`, which filters out revisions in the
-input set if they're ancestors of other revisions in the set.
+`git log --ancestry-path`), range (`foo..bar`, same as Git's). See
+[the revset documentation](revsets.md) for all revset operators and functions.
 
 ## Conflicts