printk: Move the printk() kerneldoc comment to its new home

printk: Move the printk() kerneldoc comment to its new home

Commit 337015573718 ("printk: Userspace format indexing support") turned
printk() into a macro, but left the kerneldoc comment for it with the (now)
_printk() function, resulting in this docs-build warning:

  kernel/printk/printk.c:1: warning: 'printk' not found

Move the kerneldoc comment back next to the (now) macro it's meant to
describe and have the docs build find it there.

Fixes: 337015573718b161 ("printk: Userspace format indexing support")
Signed-off-by: Jonathan Corbet <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
Link: https://lore.kernel.org/r/[email protected]