Update window_functions.md

[soerenwolfers]

• Fix a column with containing incremental to just say a column to enumerate
• Clarify that AGG(x ORDER BY y) OVER (ORDER BY z) is not supported
• Clarify that AGG(x) OVER (ORDER BY y) does behave like AGG(x ORDER BY y) OVER (ORDER BY y) (see hawkfish's confirmation at Add support for ORDER BY within window functions for MODE duckdb#14996)
• Claim that ordering is not well-defined without an ORDER BY clause -- although from experiments it seems like the builtin table order (rowid) is used by default? Please confirm, that would be super useful if true.

[szarnyasg]

Thanks! I adjusted the URLs but otherwise this is good to go in. It reminded me of the fact that people sometimes use row_number where rowid would be simpler and faster, so I'll add that as a tip later.