More test updates - add readFE/readFF when writing sync/single.

I used readFF() on a sync, instead of readFE, in a few cases
where the computation is over, to say ''read the value and
keep it there just in case''.

I also slightly updated the sync/single primer with sync/single
generic formals, mention we can't just write them, added
some clearly missing newlines.
Updated the error messages, after consulting with Brad.