Update docs for getattr() behavior with a default value

Summary: `getattr()` behaves differently depending on the presence of a default value, clarify behavior in the docs.

Reviewed By: ndmitchell, IanChilds

Differential Revision: D57437796

fbshipit-source-id: d06de75f6ffa9f2672f9f2d1d8c86d37c2cf8967

starlark/src/stdlib/funcs/other.rs

@@ -342,6 +342,9 @@ pub(crate) fn register_other(builder: &mut GlobalsBuilder) {
     ///
     /// `getattr(x, "f")` is equivalent to `x.f`.
     ///
+    /// `getattr(x, "f", d)` is equivalent to `x.f if hasattr(x, "f") else d`
+    /// and will never raise an error.
+    ///
     /// ```
     /// # starlark::assert::all_true(r#"
     /// getattr("banana", "split")("a") == ["b", "n", "n", ""] # equivalent to "banana".split("a")