[docs] Include a note to turn on the feature correctly.

Bug: chromium:1467464, b:308095975
Change-Id: I59bf0b7dae5848f0b70ba686bc79669da728fea7
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5129628
Commit-Queue: Benedikt Meurer <[email protected]>
Auto-Submit: Benedikt Meurer <[email protected]>
Reviewed-by: Danil Somsikov <[email protected]>
Commit-Queue: Danil Somsikov <[email protected]>
diff --git a/docs/visual_logging.md b/docs/visual_logging.md
index a6ce219..f459d06 100644
--- a/docs/visual_logging.md
+++ b/docs/visual_logging.md
@@ -8,6 +8,9 @@
 we can improve the product. This includes understanding what users are seeing,
 what they are interacting with, and how they are using different features.
 
+To turn on the logging, you need to pass `--enable-features=DevToolsVeLogging`
+as command line flag to Chrome.
+
 ## General approach
 
 We log impressions and interactions for a subtree of the actual DevTools
@@ -190,3 +193,6 @@
 structure look like. To do that, call `setVeDebuggingEnabled(true)` in DevTools
 on DevTools. This will add red outline to each visual element and will show the
 details of logging config for an element and all its ancestors on hover.
+
+**Note:** This will only work if you invoked Chrome with the command line flag
+`--enable-features=DevToolsVeLogging`. Otherwise you won't see any red lines.