docs: document graceful job kill/stop and job.terminate_interval (bug-1957)

- src/docs/xce.md: explicit signal and force kill are REST-only (signal,
  force_kill); xcecli has no signal option yet.
- src/docs/deployment.md: app.yaml row lists the job block
  (max_array_size, terminate_interval) and trusted proxies.
- xcecli jobs kill/stop/resume help describes the signal sequences.
- REST: control endpoint annotations and the signal/force_kill model fields
  describe the behaviour (swagger output not regenerated: swag fails on an
  unrelated annotation).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/xcecli/cmd/jobs/controller.go

@@ -151,6 +151,11 @@ func NewKillJobsCmd(configManager *config.ConfigManager) *cobra.Command {
 		Use:   "kill",
 		Short: "Kill jobs",
 		Long: `Kill jobs by job IDs or filters.
+
+The job gets SIGINT, then SIGTERM, then SIGKILL, one interval apart (the
+API server's job.terminate_interval, default 10 s), each only while it is
+still running, so it can catch SIGINT/SIGTERM and clean up.
+
 Examples:
   xce jobs kill jobid1,jobid2,...      # kill jobs by job IDs
   xce jobs kill <array-id>[k]          # kill element k of an array
@@ -190,6 +195,11 @@ func NewStopJobsCmd(configManager *config.ConfigManager) *cobra.Command {
 		Use:   "stop",
 		Short: "Stop jobs",
 		Long: `Stop jobs by job IDs or filters.
+
+The job gets SIGTSTP first, so it can catch it, prepare and stop itself;
+SIGSTOP follows after the interval (job.terminate_interval, default 10 s)
+if it has not stopped.
+
 Examples:
   xce jobs stop jobid1,jobid2,...      # stop jobs by job IDs
   xce jobs stop <array-id>[k]          # stop element k of an array
@@ -229,6 +239,9 @@ func NewResumeJobsCmd(configManager *config.ConfigManager) *cobra.Command {
 		Use:   "resume",
 		Short: "Resume jobs",
 		Long: `Resume jobs by job IDs or filters.
+
+The job gets SIGCONT; a stop still in progress is cancelled.
+
 Examples:
   xce jobs resume jobid1,jobid2,...      # resume jobs by job IDs
   xce jobs resume <array-id>[k]          # resume element k of an array