Add link to Further Rule Documentation

Update https://bazel.build/rules/lib/rule to point to https://bazel.build/extending/rules for more information about `rule(...)`.

Issue reference: bazelbuild#16373

Closes bazelbuild#16372.

PiperOrigin-RevId: 490250575
Change-Id: Ib5e926421d73ce9843ed71d902bd0f6d20b06709

src/main/java/com/google/devtools/build/lib/packages/RuleFunction.java

@@ -25,7 +25,8 @@
     doc =
         "A callable value representing the type of a native or Starlark rule. Calling the value"
             + " during evaluation of a package's BUILD file creates an instance of the rule and"
-            + " adds it to the package's target set.")
+            + " adds it to the package's target set. For more information, visit this page about"
+            + "<a href ='https://bazel.build/extending/rules'>Rules</a>.")
 public interface RuleFunction extends StarlarkCallable {
   RuleClass getRuleClass();
 }