docs: Improve grammar in Userspace API/fwctl

docs: Improve grammar in Userspace API/fwctl

Fix typos and improve grammar in the documentation for
fwctl subsystem.

Use the word user space consistently, instead of having
two variants (user space vs. userspace).

Change wording of denied behaviour to be disallowed
behaviour when describing the interface.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Hanne-Lotta Mäenpää <hannelotta@gmail.com>
Link: https://lore.kernel.org/r/20250522115255.137450-3-hannelotta@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>