devres: Improve devm_kasprintf()/kvasprintf() support

  - Add devm_kasprintf()/kvasprintf(), introduced by commit
    75f2a4ea ("devres: Add
    devm_kasprintf and devm_kvasprintf API"), to
    Documentation/driver-model/devres.txt,
  - Improve kernel doc: the string is not an existing formatted string,
    but is formatted into the newly-allocated buffer,
  - Add a __printf() annotation to devm_kasprintf(), so the compiler
    will verify the format string argument types.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>