Doc patch on psql output formats

Hi,

psql's documentation has this mention about output formats:
"Unique abbreviations are allowed. (That would mean one letter is enough.)"

but "one letter is enough" is not true since 9.3 that added
"latex-longtable" sharing the same start as "latex", and then
9.5 added "asciidoc" with the same first letter as "aligned".

When a non-unique abbreviation is used, psql uses the first
match in an arbitrary order defined in do_pset() by
a cascade of pg_strncasecmp().
(the recent commit add9182e reorders them alphabetically
but it turns out that it doesn't change the relative positions
of "aligned" / "asciidoc", or "latex" / "latex-longtables"
so \pset format a and \pset format l will continue to
select "aligned" and "latex" as before).

Anyway, "Unique abbreviations are allowed" deserves to
be changed as well.

PFA a doc patch to say simply "Abbreviations are allowed".

Best regards,
--
Daniel Vérité
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Twitter: @DanielVerite