doc: Clarify collation requirements for base32hex sortability.

doc: Clarify collation requirements for base32hex sortability.

While fixing the base32hex UUID sortability test in commit
89210037a0a, it turned out that the expected lexicographical order is
only maintained under the C collation (or an equivalent byte-wise
collation). Natural language collations may employ different rules,
breaking the sortability.

This commit updates the documentation to explicitly state that
base32hex is "byte-wise sortable", ensuring users do not fall into the
trap of using natural language collations when querying their encoded
data.

Co-Authored-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CAD21AoAwX1D6baSGuQXm0mzPXPWB07kgaoaaahjNHHenbdY24A@mail.gmail.com