Update src/triagebot/behind-upstream.md

Co-authored-by: Urgau <[email protected]>

Author: xizheyin

src/triagebot/behind-upstream.md

@@ -1,13 +1,15 @@
 # Behind Upstream
 
-This is what happens when a PR's code is based on a very old commit from an upstream branch:
+This handler checks if a PR is based on an *X* days old commit.
+
+## Context
+
+When a PR's code is based on a very old commit from an upstream branch:
 It passes when tested locally, but fails when the PR is submitted for testing through CI.
 
 This is because the CI applies the commit patches to the current upstream branch,
 which may have new test cases, so it won't pass. We need to rebase the PR to the nearest upstream branch.
 
-This option checks if a PR is based on an older branch upstream.
-
 ## Configuration
 
 This feature is enabled on a repository by having a `[behind-upstream]` table in `triagebot.toml`: