Add reference to swiftlang/swift-docc#1218

Author: ffried

.github/workflows/swift-generate-and-publish-docs.yml

@@ -212,7 +212,7 @@ jobs:
           OUTPUT_PATH: ${{ inputs.repository }}-combined.doccarchive
         run: |
           # We could use `xargs -J` in POSIX - but GNU xargs doesn't have -J...
-          # The exec mkdir is currently needed when merging a single archive. Otherwise `docc merge` fails.
+          # The exec mkdir is currently needed when merging a single archive. Otherwise `docc merge` fails: https://github.com/swiftlang/swift-docc/pull/1218
           find artifacts -type d -name '*.doccarchive' -exec mkdir -p '{}/data/tutorials' '{}/data/documentation' \; -print0 \
             | xargs --null --no-run-if-empty \
               sh -c "'$(which docc || xcrun --find docc)' merge \"\$@\" --output-path '${OUTPUT_PATH}'" sh