Add doc for triagebot [behind-upstream] handler

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

src/SUMMARY.md

@@ -16,6 +16,7 @@
     - [PR Assignment](./triagebot/pr-assignment.md)
     - [Tracking PR assignment](./triagebot/pr-assignment-tracking.md)
     - [Autolabels](./triagebot/autolabels.md)
+    - [Behind Upstream](./triagebot/behind-upstream.md)
     - [Canonicalize Issue Links](./triagebot/canonicalize-issue-links.md)
     - [Close](./triagebot/close.md)
     - [Documentation Updates](./triagebot/doc-updates.md)

src/triagebot/behind-upstream.md

@@ -0,0 +1,22 @@
+# Behind Upstream
+
+Sometimes a PR may be based on a very old commit in the upstream branch, and when submitting the PR test, CI will apply the patch to the current upstream repository, which may conflict with the new test, so the branch needs to be updated in time.
+
+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`:
+
+```toml
+[behind-upstream]
+```
+or, you can set the day threshold,
+```toml
+[behind-upstream]
+days-threshold = 7
+```
+
+## Implementation
+
+See [`src/handlers/check_commits/behind_upstream.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/check_commits/behind_upstream.rs).