add descriptive comment for TIF_MEMDIE task flag declaration.

arch/alpha/include/asm/thread_info.h

arch/arm/include/asm/thread_info.h

arch/avr32/include/asm/thread_info.h

arch/blackfin/include/asm/thread_info.h

arch/cris/include/asm/thread_info.h

arch/frv/include/asm/thread_info.h

arch/h8300/include/asm/thread_info.h

arch/ia64/include/asm/thread_info.h

arch/m32r/include/asm/thread_info.h

arch/m68k/include/asm/thread_info_mm.h

arch/m68k/include/asm/thread_info_no.h

arch/microblaze/include/asm/thread_info.h

arch/mips/include/asm/thread_info.h

arch/mn10300/include/asm/thread_info.h

arch/parisc/include/asm/thread_info.h

arch/powerpc/include/asm/thread_info.h

arch/s390/include/asm/thread_info.h

arch/score/include/asm/thread_info.h

arch/sh/include/asm/thread_info.h

arch/sparc/include/asm/thread_info_32.h