docs: be more specific about supported schedule granularity (#11383)

docs/usage/noise-reduction.md

@@ -101,6 +101,7 @@ Or perhaps at least weekly:
 If you're wondering what is supported and not, under the hood, the schedule is parsed using [@breejs/later](https://github.com/breejs/later) using the `later.parse.text(scheduleString)` API.
 Read the parser documentation at [breejs.github.io/later/parsers.html#text](https://breejs.github.io/later/parsers.html#text).
 Renovate does not support scheduled minutes or "at an exact time" granularity.
+Granularity must be at least one hour.
 
 ## Automerging