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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/907ac2ca30bbe7e91959f42f8e420191bb1f2708

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