Merge pull request #172164 from raboof/docs-clarify-mass-rebuild

docs: clarify what a 'mass rebuild' is

doc/contributing/submitting-changes.chapter.md

@@ -236,7 +236,7 @@ The `master` branch is the main development branch. It should only see non-break
 
 ### Staging branch {#submitting-changes-staging-branch}
 
-The `staging` branch is a development branch where mass-rebuilds go. It should only see non-breaking mass-rebuild commits. That means it is not to be used for testing, and changes must have been well tested already. If the branch is already in a broken state, please refrain from adding extra new breakages.
+The `staging` branch is a development branch where mass-rebuilds go. Mass rebuilds are commits that cause rebuilds for many packages, like more than 500 (or perhaps, if it's 'light' packages, 1000). It should only see non-breaking mass-rebuild commits. That means it is not to be used for testing, and changes must have been well tested already. If the branch is already in a broken state, please refrain from adding extra new breakages.
 
 ### Staging-next branch {#submitting-changes-staging-next-branch}