Add documentation for triagebot check-commits = "uncanonicalized"

Author: Urgau

src/triagebot/issue-links.md

@@ -21,11 +21,23 @@ This feature is enabled on a repository by having a `[issue-links]` table in `tr
 
 ### Without commits checking
 
+Some repositories may not need, or may prefer not, to warn about issue links in commit messages. This behavior can be disabled with the `check-commits` configuration option:
+
 ```toml
 [issue-links]
 check-commits = false # defaults to true
 ```
 
+#### Only check uncanonicalized issue links
+
+For subtrees (or embedded repositories), it is recommended to still check for *uncanonicalized* issue links (e.g. `#132` instead of `rust-lang/cargo#123`). 
+This prevents links from resolving to the wrong repository when the subtree is merged upstream while still allowing issue links in commits.
+
+```toml
+[issue-links]
+check-commits = "uncanonicalized" # for subtrees
+```
+
 ## Implementation
 
 See [`src/handlers/issue_links.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/issue_links.rs) and [`src/handlers/check_commits/issue_links.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/check_commits/issue_links.rs).