docs: add Juju version markers for start/stop checks (#1592)

The version of Pebble that includes the start/stop check functionality
has been [merged into Juju](juju/juju#19037), so
should be in the next Juju release. 3.6.3 is in candidate and 3.6.4 is
edge, so I think that means it'll be in 3.6.4, so set that as the
marker.


[Preview](https://ops--1592.org.readthedocs.build/en/1592/reference/ops.html#ops.Container.start_checks)

ops/model.py

@@ -2506,6 +2506,8 @@ def get_check(self, check_name: str) -> pebble.CheckInfo:
     def start_checks(self, *check_names: str) -> List[str]:
         """Start given check(s) by name.
 
+        .. jujuadded:: 3.6.4
+
         Returns:
             A list of check names that were started. Checks that were already
             running will not be included.
@@ -2518,6 +2520,8 @@ def start_checks(self, *check_names: str) -> List[str]:
     def stop_checks(self, *check_names: str) -> List[str]:
         """Stop given check(s) by name.
 
+        .. jujuadded:: 3.6.4
+
         Returns:
             A list of check names that were stopped. Checks that were already
             inactive will not be included.

ops/pebble.py

@@ -3101,6 +3101,8 @@ def get_checks(
     def start_checks(self, checks: Iterable[str]) -> List[str]:
         """Start checks by name.
 
+        .. jujuadded:: 3.6.4
+
         Args:
             checks: Non-empty list of checks to start.
 
@@ -3113,6 +3115,8 @@ def start_checks(self, checks: Iterable[str]) -> List[str]:
     def stop_checks(self, checks: Iterable[str]) -> List[str]:
         """Stop checks by name.
 
+        .. jujuadded:: 3.6.4
+
         Args:
             checks: Non-empty list of checks to stop.