The man page states that it isn't a complete
Nvi reference.
There was a Vi reference document in section 13 of the Unix User's Supplemental Documents (USD) collection, which various Unix systems included (under /usr/share, typically). This reference was never included with OpenBSD.
That said, the Nvi source tree came from NetBSD, and the reference document
still exists in the OpenBSD CVS repository from that initial import 21 years ago.
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To my knowledge, the flags are g: global, c: confirm, and i: ignore