Include note about Markdown ignore comments (#17060)

Include not about Markdown ignore comments

docs/ignore.md

@@ -145,6 +145,8 @@ This type of ignore is only allowed to be used in top-level and aimed to disable
 <!-- prettier-ignore-end -->
 ```
 
+**Important:** You must have a blank line before `<!-- prettier-ignore-start -->` and `<!-- prettier-ignore-end -->` for Prettier to recognize the comments.
+
 ## YAML
 
 To ignore a part of a YAML file, `# prettier-ignore` should be placed on the line immediately above the ignored node: