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_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9af075c4e27ebe8d18e9b4f4bbd6b88ae73bd61a

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