gitrepo: document limitation of spec.ref.name with Azure Devops

Signed-off-by: Sanskar Jaiswal <[email protected]>

Author: aryan9600

docs/spec/v1/gitrepositories.md

@@ -319,6 +319,12 @@ Valid examples are: `refs/heads/main`, `refs/tags/v0.1.0`, `refs/pull/420/head`,
 This field takes precedence over [`.branch`](#branch-example),
 [`.tag`](#tag-example), and [`.semver`](#semver-example).
 
+**Note:** Azure Devops and AWS CodeCommit don't support fetching the HEAD of
+a PR. While Azure Devops lets you fetch the merge commit that will be created
+after merging a PR (using `refs/pull/<id>/merge`), this field can only be used
+to fetch references that exist in the current state of the Git repository and
+not references that will be created in the future.
+
 #### Commit example
 
 To Git checkout a specified commit, use `.spec.ref.commit`: