landlock: Fix grammar issues in documentation

Improve user space and kernel documentation.

Signed-off-by: Daniel Burgener <dburgener@linux.microsoft.com>
Link: https://lore.kernel.org/r/20241015172647.2007644-1-dburgener@linux.microsoft.com


[mic: Extend commit message, reword ptrace restriction as discussed in
the thread]
Signed-off-by: Mickaël Salaün <mic@digikod.net>