add `pattern_visitor` to api reference

stix2/__init__.py

@@ -9,6 +9,7 @@
    exceptions
    markings
    parsing
+   pattern_visitor
    patterns
    properties
    utils

stix2/pattern_visitor.py

@@ -1,3 +1,5 @@
+"""STIX2 classes and methods to generate AST from patterns"""
+
 import importlib
 import inspect