libm: explain why some funcs are not implemented (yet)
libm: explain why some funcs are not implemented (yet)

Signed-off-by: Bernhard Reutner-Fischer <ce1ac9e9ad16abccd7821f371ad381197b4768ac@gmail.com>

groundwater/uClibc,mephi42/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,majek/uclibc-vx32,hjl-tools/uClibc,kraj/uClibc,ysat0/uClibc,ddcc/klee-uclibc-0.9.33.2,kraj/uclibc-ng,skristiansson/uClibc-or1k,foss-xtensa/uClibc,ffainelli/uClibc,ddcc/klee-uclibc-0.9.33.2,foss-xtensa/uClibc,hwoarang/uClibc,OpenInkpot-archive/iplinux-uclibc,hwoarang/uClibc,atgreen/uClibc-moxie,brgl/uclibc-ng,ndmsystems/uClibc,brgl/uclibc-ng,kraj/uclibc-ng,majek/uclibc-vx32,groundwater/uClibc,majek/uclibc-vx32,ysat0/uClibc,ffainelli/uClibc,ysat0/uClibc,czankel/xtensa-uclibc,wbx-github/uclibc-ng,OpenInkpot-archive/iplinux-uclibc,majek/uclibc-vx32,m-labs/uclibc-lm32,OpenInkpot-archive/iplinux-uclibc,m-labs/uclibc-lm32,wbx-github/uclibc-ng,OpenInkpot-archive/iplinux-uclibc,czankel/xtensa-uclibc,foss-for-synopsys-dwc-arc-processors/uClibc,wbx-github/uclibc-ng,ddcc/klee-uclibc-0.9.33.2,waweber/uclibc-clang,wbx-github/uclibc-ng,m-labs/uclibc-lm32,waweber/uclibc-clang,hwoarang/uClibc,groundwater/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,hjl-tools/uClibc,waweber/uclibc-clang,ffainelli/uClibc,waweber/uclibc-clang,kraj/uClibc,hwoarang/uClibc,skristiansson/uClibc-or1k,kraj/uClibc,skristiansson/uClibc-or1k,czankel/xtensa-uclibc,atgreen/uClibc-moxie,ysat0/uClibc,atgreen/uClibc-moxie,kraj/uclibc-ng,ndmsystems/uClibc,brgl/uclibc-ng,atgreen/uClibc-moxie,groundwater/uClibc,ndmsystems/uClibc,brgl/uclibc-ng,ndmsystems/uClibc,hjl-tools/uClibc,mephi42/uClibc,hjl-tools/uClibc,m-labs/uclibc-lm32,foss-xtensa/uClibc,kraj/uClibc,skristiansson/uClibc-or1k,foss-xtensa/uClibc,ffainelli/uClibc,mephi42/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,groundwater/uClibc,kraj/uclibc-ng,hjl-tools/uClibc,czankel/xtensa-uclibc,ffainelli/uClibc,mephi42/uClibc,ddcc/klee-uclibc-0.9.33.2