ci: add comment explaining fetch-depth: 0 in lint workflow

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

.github/workflows/lint.yml

@@ -18,7 +18,7 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
-          fetch-depth: 0
+          fetch-depth: 0 # needed for merge-base used in lint:links-in-modified-files
 
       - name: Setup mise
         uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1