docs: filesystems: vfs: actualize struct file_system_type description

docs: filesystems: vfs: actualize struct file_system_type description

Added descriptions for:
- fscontext API ('init_fs_context' method, 'parameters' field)
- 'fs_supers' field

Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230313130718.253708-2-aleksandr.mikhalitsyn@canonical.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>