pgsql: Further clarify documentation for the initcap function

Further clarify documentation for the initcap function

This is a follow-up for commit c2c2c7e225. It further clarifies the
following in the initcap function documentation:

* Document that title case is used for digraphs in specific locales,
* Reference particular ICU function used,
* Add note about the purpose of the function.

Discussion: https://postgr.es/m/804cc10ef95d4d3b298e76b181fd9437%40postgrespro.ru
Author: Oleg Tselebrovskiy <o(dot)tselebrovskiy(at)postgrespro(dot)ru>
Co-authored-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Reviewed-by: Jeff Davis <pgsql(at)j-davis(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f8ce9ed220b5edd8ba751706bbcdbcc5b64be66e

Modified Files
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doc/src/sgml/func/func-string.sgml | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)