sys: printk: restore the API in the Doxygen output

Doxygen does not define CONFIG_PRINTK, so it only ever sees the
CONFIG_PRINTK=n stubs, and hiding those behind @cond left the printk()
documentation with nothing to attach to. It landed on snprintk()
instead, which warns about undocumented parameters, and the rest of the
header dropped out of the API documentation entirely.

Add __DOXYGEN__ to the condition so the real prototypes are what gets
documented, and document snprintk(), vsnprintk() and vprintk(), which
never had documentation of their own.

Assisted-by: Claude:cursor-5
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>