docs(labels): note that labels are only added at PR creation time (#1…

…7234)

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

docs/usage/configuration-options.md

@@ -1316,8 +1316,8 @@ Use this configuration option for shared config across npm/Yarn/pnpm and meteor
 
 ## labels
 
-By default, Renovate won't add any labels to its PRs.
-If you want Renovate to do so then define a `labels` array of one or more label strings.
+By default, Renovate won't add any labels to PRs.
+If you want Renovate to add labels to PRs it creates then define a `labels` array of one or more label strings.
 If you want the same label(s) for every PR then you can configure it at the top level of config.
 However you can also fully override them on a per-package basis.
 
@@ -1337,6 +1337,14 @@ Consider this example:
 
 With the above config, every PR raised by Renovate will have the label `dependencies` while PRs containing `eslint`-related packages will instead have the label `linting`.
 
+Renovate only adds labels when it creates the PR, which means:
+
+- If you remove labels which Renovate added, it won't re-apply them
+- If you change your config, the new/changed labels are not applied to any open PRs
+
+The `labels` array is non-mergeable, meaning if multiple `packageRules` match then Renovate uses the last value for `labels`.
+If you want to add/combine labels, use the `addLabels` config option, which is mergeable.
+
 ## lockFileMaintenance
 
 This feature can be used to refresh lock files and keep them up-to-date.