[JIT] add freeze to docs (#47120)

Summary:
freeze was temporarily renamed to _freeze in a reorg, and then removed from doc [here](https://github.com/pytorch/pytorch/pull/43473). add it back to docs.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/47120

Reviewed By: suo

Differential Revision: D24650712

Pulled By: eellison

fbshipit-source-id: 399e31586b8093de66937ba1266007ee291f509e
diff --git a/docs/source/jit.rst b/docs/source/jit.rst
index 50e2478..710f2f9 100644
--- a/docs/source/jit.rst
+++ b/docs/source/jit.rst
@@ -51,6 +51,7 @@
     wait
     ScriptModule
     ScriptFunction
+    freeze
     save
     load
     ignore