VOP_ALLOCATE: Update man page for Commit f0c9847a6c47

Commit f0c9847a6c47 added the ioflag and cred arguments to
VOP_ALLOCATE() for NFSv4.2 server support. This patch updates
the man page for these arguments.

Reviewed by: khng, gbe
Differential Revision: https://reviews.freebsd.org/D32898