Generate the doc for bxl utils functions

Summary:
Generate the doc for bxl utils functions

With this, user can search the utils functions on our doc page.
With future adding functions, it will also appear here.

We can also add the docs for build rule's util functions

 {F1974926634}

Reviewed By: JakobDegen

Differential Revision: D69126057

fbshipit-source-id: 5a29e1121e16105d5a8001386815f97afa566d51

starlark/src/docs.rs

@@ -61,6 +61,24 @@ pub struct DocModule {
     pub members: SmallMap<String, DocItem>,
 }
 
+impl DocModule {
+    pub fn filter<P>(self, mut predicate: P) -> Self
+    where
+        P: FnMut(&(String, DocItem)) -> bool,
+    {
+        let members = self
+            .members
+            .into_iter()
+            .filter(|member| predicate(member))
+            .collect();
+
+        Self {
+            docs: self.docs,
+            members,
+        }
+    }
+}
+
 /// Documents a single function.
 #[derive(Debug, Clone, PartialEq, Default, Allocative)]
 pub struct DocFunction {

starlark/src/environment/modules.rs

@@ -235,7 +235,11 @@ impl FrozenModule {
     pub fn documentation(&self) -> DocModule {
         let members = self
             .all_items()
-            .filter(|n| Module::default_visibility(n.0.as_str()) == Visibility::Public)
+            .filter(|n| {
+                // We only want to show public symbols in the documentation
+                self.get_any_visibility_option(n.0.as_str())
+                    .map_or(false, |(_, vis)| vis == Visibility::Public)
+            })
             // FIXME(JakobDegen): Throws out information
             .map(|(k, v)| {
                 (