Improve UX for cleanup policy regexp anchors

For Mention automatic anchors for tag cleanup polic... (#466252 - closed) a info alert was added in the clean up settings page.

@bonnie-tsang suggested UX improvement:

If we want to improve the user experience in the future, I suggest we can explore using prefixes \A and suffixes \Z to provide immediate context about what is required or not required in the input field. Just like the classic prefix https:// in a URL field clarifies that the user does not need to input this part.