BUG #19830: Mention that UNIQUE on a nondeterministic ICU collation treats ignorable characters as duplicates

The following bug has been logged on the website:

Bug reference: 19830
Logged by: aswini kumar tummala
Email address: aswin(dot)tummala(at)gmail(dot)com
PostgreSQL version: 19rc1
Operating system: Windows, MacOs
Description:

Hi,

When migrating from SQL Server, we found that a UNIQUE constraint on a
column with und-u-ks-level2 (deterministic = false)
rejects 'ACME' || U&'\200B' as a duplicate of 'ACME'. The same data loads
fine into a citext column. Section 23.2.3.1 does show
U+2063 being ignored, but it doesn't say what that means for constraints.
A short note under "Nondeterministic Collations" would
have saved us time. It could say that ignorable code points (zero-width
space, soft hyphen, BOM, most control characters) don't
count toward uniqueness, and that this differs from citext.

Reproduced on 18.6 and 19 RC1.

Thanks,
Aswini Kumar Tummala