Doc: remove claim that all \pset format options are unique in 1 letter.

Doc: remove claim that all \pset format options are unique in 1 letter.

This hasn't been correct since 9.3 added "latex-longtable".

I left the phraseology "Unique abbreviations are allowed" alone.
It's correct as far as it goes, and we are studiously refraining
from specifying exactly what happens if you give a non-unique
abbreviation.  (The answer in the back branches is "you get a
backwards-compatible choice", and the answer in HEAD will shortly
be "you get an error", but there seems no need to mention such
details here.)

Daniel Vérité

Discussion: https://postgr.es/m/cb7e1caf-3ea6-450d-af28-f524903a030c@manitou-mail.org