[easy] missing this_host in docs

Pull Request resolved: #1461

Differential Revision: [D84097597](https://our.internmc.facebook.com/intern/diff/D84097597/)

**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D84097597/)!
ghstack-source-id: 314896719

Author: zdevito

docs/source/api/monarch.actor.rst

@@ -27,6 +27,8 @@ are launch across hosts. HostMesh represents a mesh of hosts. ProcMesh is a mesh
 
 .. autofunction:: get_or_spawn_controller
 
+.. autofunction:: this_host
+
 
 Defining Actors
 ===============