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.tselebrovskiy@postgrespro.ru>
Co-authored-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>