Leave attributes starting with underscore undocumented.

RELNOTES: None.
PiperOrigin-RevId: 210590508

src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java

diff --git a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
index a01d018..8f1eb49 100644
--- a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
+++ b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
@@ -81,6 +81,7 @@
 
     attrsMapBuilder.put("name", IMPLICIT_NAME_ATTRIBUTE_DESCRIPTOR);
     attrInfos = attrsMapBuilder.build().entrySet().stream()
+        .filter(entry -> !entry.getKey().startsWith("_"))
         .map(entry -> new AttributeInfo(
             entry.getKey(),
             entry.getValue().getDocString(),