Xml doc comment cleanup

[abelbraaksma]

Just some cleanup from discussions in #219.

@bartelink feel free to commit to this branch if you have better wording for some doc comments. You mentioned yesterday you wanted to give some of it another look.

[abelbraaksma]

I just noticed that some functions don't have a <returns> element in the doc xml (like length or lengthBy functions). I'll check that tomorrow (well, later in the AM of today, that is, unless you beat me to it ;).

[bartelink]

Hm not sure I have push rights (have pushed to https://github.com/bartelink/FSharp.Control.TaskSeq/tree/doc-cleanup instead for now)

[abelbraaksma]

@bartelink can you try again? I upped your rights, you should be allowed to push (but not sure that allows you to push on other branches). Otherwise, we can always take a merge from your fork.

[abelbraaksma]

Just this one for now, will look more into the rest.

[abelbraaksma]

These changes form an integral part of this continuation-PR: #234. With few exceptions, all changes have been accepted (there, commits were squashed for cleaner history).