Docs: remove mentions of fdformat from util-linux

Since util-linux commit 13b26e3c36d1
("fdformat: remove command from default build")
the fdformat tool is not built anymore by default.
As a result it is not packaged anymore by distributions and therefore
not usable by users.

Instead mention the "mount" command as more likely to be present
alternative.

Also drop the reference to fdformat from the list of features of new
versions of util-linux.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231220-docs-fdformat-v1-1-0d05279e5d83@weissschuh.net