It may be a trivial thing but I am not sure we need to mention case insensitivity here, because all keywords and unquoted identifiers are case-insensitive in PostgreSQL and it is not specific to NULL.
But it is neither a keyword nor an identifier. It behaves more like: SELECT 1 as one; A constant, which have no implied rules - mainly because numbers don't have case. Which suggests adding some specific mention there - and also probably need to bring up it and its "untyped" nature in the syntax chapter, probably here:
Also, I found the other parts of the documentation use "case-insensitive" in which words are joined with hyphen, so I wonder it is better to use the same form if we leave the description.
Typo on my part, fixed.
I'm not totally against just letting this content be assumed to be learned from elsewhere in the documentation but it also seems reasonable to include. I'm going to leave it for now.