fs: turn some comments into kernel-doc

While reviewing ./include/linux/fs.h, I noticed that three comments can
actually be turned into kernel-doc comments. This allows to check the
consistency between the descriptions and the functions' signatures in
case they may change in the future.

A quick validation with the consistency check:

  ./scripts/kernel-doc -none include/linux/fs.h

currently reports no issues in this file.

Link: https://lore.kernel.org/r/20210204180059.28360-2-lukas.bulwahn@gmail.com


Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>