docs: max-runtime is not valid on job-exec

QUICK_REFERENCE.md told operators to add max-runtime to a [job-exec]
section. The key exists on RunJob, RunServiceJob and [global], but ExecJob
has no such field, so the documented configuration is rejected.

Point at the global default instead, and mark the block as ini rather than
bash so a config-parsing check can see it — the snippet was invisible to
the docs gate precisely because of the fence language.

Found by the documentation-drift gate being built for this repo, which is
the kind of defect it exists to catch.

Signed-off-by: Sebastian Mendel <info@sebastianmendel.de>

Author: CybotTM

docs/QUICK_REFERENCE.md

@@ -402,14 +402,22 @@ go tool pprof cpu.prof
 ```
 
 ### Jobs Overlapping
-```bash
+```ini
 # Add no-overlap to job config
 [job-exec "long-task"]
 schedule = @hourly
 container = worker
 command = /scripts/task.sh
 no-overlap = true               # ← Add this
-max-runtime = 50m               # ← And timeout
+```
+
+`max-runtime` is not available on `job-exec`; it applies to `job-run`,
+`job-service-run` and `[global]`. To bound an exec job, set the global
+default:
+
+```ini
+[global]
+max-runtime = 50m
 ```
 
 ## Performance Tips