Add documentation

About operator config and adjust wording in manifest docs

docs/reference/cluster_manifest.md

@@ -96,12 +96,18 @@ Those are parameters grouped directly under  the `spec` key in the manifest.
    that should be assigned to the cluster pods. When not specified, the value
    is taken from the `pod_priority_class_name` operator parameter, if not set
    then the default priority class is taken. The priority class itself must be defined in advance.
-   
+
 * **shmVolume**
   Start a database pod without limitations on shm memory. By default docker
-  limit `/dev/shm` to `64M`, which could be not enough if PostgreSQL uses
-  parallel workers heavily. If this option is enabled, to the target database
-  pod will be mounted a new tmpfs volume to remove this limitation.
+  limit `/dev/shm` to `64M` (see e.g. the [docker
+  issue](https://github.com/docker-library/postgres/issues/416), which could be
+  not enough if PostgreSQL uses parallel workers heavily. If this option is
+  present and value is `true`, to the target database pod will be mounted a new
+  tmpfs volume to remove this limitation. If it's not present, the decision
+  about mounting a volume will be made based on operator configuration
+  (`shm_volume`, which is `true` by default). It it's present and value is
+  `false`, then no volume will be mounted no matter how operator was configured
+  (so you can override the operator configuration).
 
 ## Postgres parameters
 

docs/reference/operator_parameters.md

@@ -224,6 +224,14 @@ CRD-based configuration.
 * **set_memory_request_to_limit**
   Set `memory_request` to `memory_limit` for all Postgres clusters (the default value is also increased). This prevents certain cases of memory overcommitment at the cost of overprovisioning memory and potential scheduling problems for containers with high memory limits due to the lack of memory on Kubernetes cluster nodes. This affects all containers (Postgres, Scalyr sidecar, and other sidecars). The default is `false`.
 
+* **shm_volume**
+  Instruct operator to start any new database pod without limitations on shm
+  memory. If this option is enabled, to the target database pod will be mounted
+  a new tmpfs volume to remove shm memory limitation (see e.g. the [docker
+  issue](https://github.com/docker-library/postgres/issues/416)). This option
+  is global for an operator object, and can be overwritten by `shmVolume`
+  parameter from Postgres manifest. The default is `true`
+
 ## Operator timeouts
 
 This set of parameters define various timeouts related to some operator