Minor tweaks to the layered.sh build script (#34300)

I struggled to understand what was going on in my failing build. Clean up some
comments, and try to make the console output a bit more helpful.

scripts/layered.sh

@@ -4,7 +4,7 @@ set -ex
 
 # Creates a layered environment with the full repo for the app and SDKs cloned
 # and linked. This gives an element-web dev environment ready to build with
-# matching branches of react-sdk's dependencies so that changes can be tested
+# matching branches of matrix-js-sdk so that changes can be tested
 # in element-web.
 
 # Note that this style is different from the recommended developer setup: this
@@ -22,13 +22,15 @@ export PR_REPO=element-web
 
 js_sdk_dep=$(jq -r '.dependencies["matrix-js-sdk"]' < $(pnpm -w root)/../apps/web/package.json)
 
-# Set up the js-sdk first (unless package.json pins a specific version)
+# Set up the js-sdk (unless package.json pins a specific version)
 if [ "$js_sdk_dep" = "github:matrix-org/matrix-js-sdk#develop" ]; then
+    echo "layered.sh: Cloning matching branch of matrix-js-sdk"
     scripts/fetchdep.sh matrix-org matrix-js-sdk develop
 
     if [ -n "$JS_SDK_GITHUB_BASE_REF" ]; then
+        echo "layered.sh: Switching js-sdk to $JS_SDK_GITHUB_BASE_REF"
         git -C matrix-js-sdk fetch --depth 1 origin $JS_SDK_GITHUB_BASE_REF
-        git -C matrix-js-sdk checkout $JS_SDK_GITHUB_BASE_REF
+        git -C matrix-js-sdk -c advice.detachedHead=false checkout $JS_SDK_GITHUB_BASE_REF
     fi
     pnpm -C matrix-js-sdk install --frozen-lockfile --ignore-scripts
 
@@ -36,5 +38,5 @@ if [ "$js_sdk_dep" = "github:matrix-org/matrix-js-sdk#develop" ]; then
     pnpm -C apps/web link ./matrix-js-sdk
     pnpm link ./matrix-js-sdk
 else
-    echo "Skipping matrix-js-sdk fetch and link as package.json pins $js_sdk_dep"
+    echo "layered.sh: Skipping matrix-js-sdk fetch and link as package.json pins $js_sdk_dep"
 fi