docs: improve generated documentation (#203)

Document both the py_binary (macro) and py_binary_rule underneath it,
like we do in other rulesets.
Same for py_test and py_test_rule
Fill in some missing docstrings.

---

### Type of change

- Documentation (updates to documentation or READMEs)

### Test plan

None

Author: alexeagle

docs/BUILD.bazel

@@ -7,14 +7,4 @@ stardoc_with_diff_test(
     bzl_library_target = "//py:defs",
 )
 
-stardoc_with_diff_test(
-    name = "py_library",
-    bzl_library_target = "//py/private:py_library",
-)
-
-stardoc_with_diff_test(
-    name = "py_binary",
-    bzl_library_target = "//py/private:py_binary",
-)
-
 update_docs(name = "update")