Add missing deploy.limits.pids (from schema 3.9), deprecate pids_limit

compose schema v3.9 added support for `deploy.limits.pids` (through docker/cli@851eeb9),
however, work on compose-spec was already in progress, which lead to changes from
the 3.9 schema to not be included.

This patch adds the missing option and mars the `services.{name}.pids_limit` as
deprecated, similar to comparable limit- and reservation options (`services.{name}.cpus`,
`services.{name}.mem_limit`, `services.{name}.mem_reservation`.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

deploy.md

@@ -143,6 +143,7 @@ services:
         limits:
           cpus: '0.50'
           memory: 50M
+          pids: 1
         reservations:
           cpus: '0.25'
           memory: 20M
@@ -156,6 +157,10 @@ services:
 
 `memory` configures a limit or reservation on the amount of memory a container can allocate, set as a string expressing a [byte value](spec.md#specifying-byte-values).
 
+#### pids
+
+`pids_limit` tunes a container’s PIDs limit, set as an integer.
+
 #### devices
 
 `devices` configures reservations of the devices a container can use. It contains a list of reservations, each set as an object with the following parameters: `capabilities`, `driver`, `count`, `device_ids` and `options`.

schema/compose-spec.json

@@ -519,7 +519,8 @@
               "type": "object",
               "properties": {
                 "cpus": {"type": ["number", "string"]},
-                "memory": {"type": "string"}
+                "memory": {"type": "string"},
+                "pids": {"type": "integer"}
               },
               "additionalProperties": false,
               "patternProperties": {"^x-": {}}

spec.md

@@ -1425,6 +1425,8 @@ Supported values are platform specific.
 
 ### pids_limit
 
+_DEPRECATED: use [deploy.reservations.pids](deploy.md#pids)_
+
 `pids_limit` tunes a container’s PIDs limit. Set to -1 for unlimited PIDs.
 
 ```yml