Logging: improve help message of LOG_PRINTK Kconfig option

Improve it so that:
- it mentions the important fact that it redirects `printk` messages
 to the logging subsystem
- it is consistent with the help messages of the other options in this
 file (i.e. it starts with "If enabled")

Signed-off-by: Ladislav Podivin <ladislav.podivin@tietoevry.com>