Make cron notation parser a proper note

What does this MR do?

There was a note in a blockquote at the top of the pipeline schedules page that mentioned the cron parser. It was rolled up into the 'Version history' (Introduced in) section and collapsed. It was fairly obscure and not searchable. I didn't know it was there until I came across a commit that changed that text.

This change makes this note a proper note that is rendered more obviously.