Cross-link for caveat purposes

Cross-link the search page and the fast-forward merge strategy
pages. Links like these keep them in sync with each other.

doc/user/project/merge_requests/fast_forward_merge.md

@@ -13,8 +13,6 @@ merge commits. In such cases, the fast-forward merge is the perfect candidate.
 With fast-forward merge requests, you can retain a linear Git history and a way
 to accept merge requests without creating merge commits.
 
-## Overview
-
 When the fast-forward merge
 ([`--ff-only`](https://git-scm.com/docs/git-merge#git-merge---ff-only)) setting
 is enabled, no merge commits are created and all merges are fast-forwarded,
@@ -22,6 +20,11 @@ which means that merging is only allowed if the branch can be fast-forwarded.
 
 When a fast-forward merge is not possible, the user is given the option to rebase.
 
+NOTE:
+Projects using the fast-forward merge strategy can't filter merge requests
+[by deployment date](../../search/index.md#filtering-merge-requests-by-environment-or-deployment-date),
+because no merge commit is created.
+
 ## Enabling fast-forward merges
 
 1. On the top bar, select **Menu > Projects** and find your project.

doc/user/search/index.md

@@ -152,6 +152,10 @@ you can type (or select from the dropdown list) the following:
 - Deployed-before
 - Deployed-after
 
+NOTE:
+Projects using a [fast-forward merge method](../project/merge_requests/fast_forward_merge.md)
+do not return results, as this method does not create a merge commit.
+
 When filtering by an environment, a dropdown list presents all environments that
 you can choose from: