Document additional PaaSTA-provided env vars (+ sort list) (#4034)

I was recently chatting with jrchu about default env vars and I realized
that there's a couple that we didn't document.

Author: nemacysts

docs/source/yelpsoa_configs.rst

@@ -60,22 +60,25 @@ specify the following options:
   * ``env``: A dictionary of environment variables that will be made available
     to the container. PaaSTA additionally will inject the following variables automatically (keep in mind all environment variables are strings in a shell):
 
-    * ``PAASTA_SERVICE``: The service name
-    * ``PAASTA_INSTANCE``: The instance name
+    * ``MESH_REGISTRATIONS`` (optional): space-separated list of registrations for a given Pod if the PaaSTA instance is mesh-routable - absent otherwise.
     * ``PAASTA_CLUSTER``: The cluster name
-    * ``PAASTA_HOST``: The hostname of the actual server the container is runnig on
-    * ``PAASTA_PORT``: The configured port the service should listen on
+    * ``PAASTA_DEPLOY_GROUP``: The `deploy group <deploy_group.html>`_ specified
     * ``PAASTA_DOCKER_IMAGE``: The docker image name
     * ``PAASTA_GIT_SHA``: The short git sha of the code the container has
-    * ``PAASTA_DEPLOY_GROUP``: The `deploy group <deploy_group.html>`_ specified
+    * ``PAASTA_HOST``: The hostname of the actual server the container is running on
+    * ``PAASTA_IMAGE_VERSION`` (optional) : The version of the docker image - only present for instances using automated redeploys or manually deployed using --image-version
+    * ``PAASTA_INSTANCE_TYPE``: The instance type of the service (e.g: tron, kubernetes, eks, etc)
+    * ``PAASTA_INSTANCE``: The instance name
+    * ``PAASTA_LAUNCHED_BY`` (optional): May not be present. If present, will have the username of the user who launched the PaaSTA container
     * ``PAASTA_MONITORING_TEAM``: The team that is configured to get alerts.
-    * ``PAASTA_LAUNCHED_BY``: May not be present. If present, will have the username of the user who launched the PaaSTA container
+    * ``PAASTA_POD_IP``: The IP of the Pod running the PaaSTA container - may not always be a routable IP
+    * ``PAASTA_PORT``: The configured port the service should listen on
     * ``PAASTA_RESOURCE_CPUS``: Number of cpus allocated to a container
-    * ``PAASTA_RESOURCE_MEM``: Amount of ram in MB allocated to a container
     * ``PAASTA_RESOURCE_DISK``: Amount of disk space in MB allocated to a container
     * ``PAASTA_RESOURCE_GPUS``: Number of GPUS (if requested) allocated to a container
-    * ``PAASTA_IMAGE_VERSION``: The version of the docker image
-    * ``PAASTA_INSTANCE_TYPE``: The instance type of the service (e.g: tron, kubernetes, eks, etc)
+    * ``PAASTA_RESOURCE_MEM``: Amount of ram in MB allocated to a container
+    * ``PAASTA_SERVICE``: The service name
+    * ``POD_NAME``: The Pod name as generated by Kubernetes.
 
 
   * ``extra_volumes``: An array of dictionaries specifying extra bind-mounts