docs/defines.txt: clarify LFS-related macros
docs/defines.txt: clarify LFS-related macros

Add description for _LARGEFILE_SOURCE, _LARGEFILE64_SOURCE,
_FILE_OFFSET_BITS and __USE_LARGEFILE, __USE_LARGEFILE64 and
__USE_FILE_OFFSET64 macros.

Signed-off-by: Max Filippov <8c5ea195856807f4f02a6e0749c87ea6b7375f38@gmail.com>

wbx-github/uclibc-ng,wbx-github/uclibc-ng,wbx-github/uclibc-ng,kraj/uclibc-ng,wbx-github/uclibc-ng,kraj/uclibc-ng,kraj/uclibc-ng,kraj/uclibc-ng