doc: kernel: k_busy_wait behavior with SYSTEM_CLOCK_SLOPPY_IDLE and PM

Added note k_busy_wait may not work when CONFIG_SYSTEM_CLOCK_SLOPPY_IDLE
and CONFIG_PM are enabled.
In such case timer used for time measurement may be disabled.

Fixes: #53522

Signed-off-by: Artur Lipowski <artur.lipowski@hidglobal.com>