forum information - how to "create a document"?

Hi I am looking in the forum resources and can't find the information about how to "Create a document"

the reason is I created an elaborate post that I wanted to be available for everybody but once someone is commenting about it - I cant change it, so I was thinking maybe there is a way to create a document that will be available here and I could revise If I needed too.

also while I am here how do I stop getting this cross signs an I write and press space bar ?

example:

thank you.