Not sure how to submit a patch, but I suggest adding these lines to what is line 74 in my /usr/share/man/man1/mupdf.1.gz in Arch Linux: .TP .B P Displays the current page number.
This is still the case on version 1.10_a-1 on Arch Linux
Fixed in commit commit 1c1454464bd0153f55204e99be45e982a85a7996 Author: Sebastian Rasmussen <sebras@gmail.com> Date: Tue Jan 12 04:35:00 2021 +0100 Bug 695317: docs: Add description of missing key bindings.