fix for :fetch_initial_posts

A warning was added for the :fetch_initial_posts setting, but their were some newlines and whitespaces missing which broke the rendering of the setting in the docs. I've added them now.

This is how it looks before this fix, as you can see, the name of the setting is missing and the bullet points aren't rendered. I now did the newlines and whitespaces the same as how it is done for the RUM indexing for full text search setting who also has a title above the warning and a bullet point below (and thus is very similar to the :fetch_initial_posts section).