Add documentation for stack commands

Also removes the `-f` flags of bundle to follow the single-letter flags
evaluation.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

docs/reference/commandline/stack_tasks.md

@@ -0,0 +1,46 @@
+<!--[metadata]>
++++
+title = "stack tasks"
+description = "The stack tasks command description and usage"
+keywords = ["stack, tasks"]
+advisory = "experimental"
+[menu.main]
+parent = "smn_cli"
++++
+<![end-metadata]-->
+
+# stack tasks (experimental)
+
+```markdown
+Usage:  docker stack tasks [OPTIONS] STACK
+
+List the tasks in the stack
+
+Options:
+  -a, --all            Display all tasks
+  -f, --filter value   Filter output based on conditions provided
+      --help           Print usage
+      --no-resolve     Do not map IDs to Names
+```
+
+Lists the tasks that are running as part of the specified stack. This
+command has to be run targeting a manager node.
+
+## Filtering
+
+The filtering flag (`-f` or `--filter`) format is a `key=value` pair. If there
+is more than one filter, then pass multiple flags (e.g. `--filter "foo=bar" --filter "bif=baz"`).
+Multiple filter flags are combined as an `OR` filter. For example,
+`-f name=redis.1 -f name=redis.7` returns both `redis.1` and `redis.7` tasks.
+
+The currently supported filters are:
+
+* [id](#id)
+* [name](#name)
+* [desired-state](#desired-state)
+
+## Related information
+
+* [stack config](stack_config.md)
+* [stack deploy](stack_deploy.md)
+* [stack rm](stack_rm.md)