[docs] search-and-replace in a column

[saulpw]

Also, an example of my confusion between the different "channels" (docs, tuts & manpage) is actually what lead me to my latest question: I looked at the "How to" about Columns and was hoping to find something about search-&-replace in a column, but could find anything there, but maybe I just didn't look properly..

Originally posted by @halloleo in https://github.com/saulpw/visidata/discussions/2221#discussioncomment-8000137