Docs/seq samples (#12165)

* Refined compareWith examples

* Split into

* Empty, duh.

* Except

* Exists

* exists2

* Filter

* Where

* Find and findback

* FindIndex

* findIndexBack

* fold

* fold2

* foldBack

* foldBack2

* groupBy

* tryHead

* Cleanup

* Cleanup

* last and tryLast

* exactlyOne to isEmpty

* indexed

* Improved a bit on foldBack

* init

* Documentation

* init infinite

* iter and iteri

* iter2

* iteri2

* length

* Fixed compareWith

* findIndexBack

* linebreaks

* fold and fold2 corrected

* One lined some examples

* Cleanup one liners

* More single lines

* Update seq.fsi
Co-authored-by: NDon Syme <dsyme@users.noreply.github.com>