pgsql: Clarify documentation for the initcap function

Clarify documentation for the initcap function

This commit documents differences in the definition of word separators for
the initcap function between libc and ICU locale providers.
Backpatch to all supported branches.

Discussion: https://postgr.es/m/804cc10ef95d4d3b298e76b181fd9437%40postgrespro.ru
Author: Oleg Tselebrovskiy <o.tselebrovskiy@postgrespro.ru>
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/276b022ec05bbd5a9639fa71e1197d53bac20075

Modified Files
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doc/src/sgml/func.sgml | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)