Add extra EINVAL information about wrong block size to read(2)/write(2)

The read system call will return EINVAL if the current file offset is
not a multiple of the block size. This also applies to write(2). Add an
entry for EINVAL about this error to both man pages.

PR: 91149
Event: Aberdeen Hackathon 2022
Differential Revision: https://reviews.freebsd.org/D24617