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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master

Details
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https://git.postgresql.org/pg/commitdiff/c2c2c7e225669e81f83a5db3f0f57131cdaa4a2d

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