To start off I'd like to say your documentation is great and very helpful. Thanks for that!
https://www.postgresql.org/docs/14/sql-select.html in the section "WINDOW clause" talks about "simple expression." I couldn't find any explaining of what actually constitutes a simple expression, not by searching for the term or checking the index.
Yeah, "simple" there isn't a definitional word, it is there to make the paragraph, which does qualify what the expression can be, read better.
You should be able to use, unless otherwise noted there or elsewhere, any of the expression building components listed in the SQL Syntax chapter. That chapter, for Window Functions, does further note that nesting window functions isn't possible.