docs: improve stabilityDays documentation (#9792)

docs/usage/configuration-options.md

@@ -2166,11 +2166,16 @@ If this setting is true then you would get one PR for webpack@v2 and one for web
 
 ## stabilityDays
 
-If this is configured to a non-zero value, and an update has a release date/timestamp available, then Renovate will check if the configured "stability days" have elapsed.
-If the days since the release is less than the configured stability days then a "pending" status check will be added to the branch.
-If enough days have passed then a passing status check will be added.
+If this is set to a non-zero value, _and_ an update contains a release timestamp header, then Renovate will check if the "stability days" have passed.
 
-There are a couple of uses for this:
+If the amount of days since the release is less than the set `stabilityDays` a "pending" status check is added to the branch.
+If enough days have passed then the "pending" status is removed, and a "passing" status check is added.
+
+Some datasources do not provide a release timestamp (in which case this feature is not compatible), and other datasources may provide a release timestamp but it's not supported by Renovate (in which case a feature request needs to be implemented).
+
+Maven users: you cannot use `stabilityDays` if a Maven source returns unreliable `last-modified` headers.
+
+There are a couple of uses for `stabilityDays`:
 
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