Lower/upper-case consistency with function names

The PostgreSQL docs mostly show function names in lowercase (e.g. [1], [2]), which seems to be the PostgreSQL-idiomatic thing to do. However, some pages show functions in upper case, e.g. COALESCE/NULLIF/GREATEST/LEAST ([3]). Is there some difference between these which warrants the case difference, or just a case of docs inconsistency?

I ran into this while trying to make Entity Framework (.NET ORM) generate more idiomatic-looking SQL. If this is just an inconsistency, it's obviously not very important.

Thanks,

Shay

[1] https://www.postgresql.org/docs/current/functions-aggregate.html

[2] https://www.postgresql.org/docs/current/datatype-datetime.html

[3] https://www.postgresql.org/docs/current/functions-conditional.html