The SQL Language part of the docs has a brief page on unique indexes. It doesn't mention the new NULLS NOT DISTINCT functionality on unique indexes and this is a good place to mention it, as it already warns the user about the old/default behavior.
I'm ok with the wording as-is, but perhaps we can phrase it as "distinct" vs "not equal", thus leaning into the syntax a bit:
By default, null values in a unique column are considered distinct, allowing multiple nulls in the column.
or maybe
By default, null values in a unique column are considered <literal>DISTINCT</literal>, allowing multiple nulls in the column.
