add comment: UOS_KeyWait returns at signal

Attention: Create new doxygen documentation after the merge!

Add a warning to the Linux specific UOS_KeyWait() documentation that signals may abort the call. Reason: The m66_irq test program uses UOS_KeyWait(); to wait until a keypress. But received MDIS signals abort the wait unexpectedly under Linux. The problem was fixed by waiting with: do { UOS_Delay(1i00); } while (UOS_KeyPressed() == (-1));

Note: m66_irq.c is not yet included in https://github.com/MEN-Mikro-Elektronik/13M066-06 but shall be in future.