manual: clarify _FILE_OFFSET_BITS Y2038 implications [BZ #34095]

manual: clarify _FILE_OFFSET_BITS Y2038 implications [BZ #34095]

The existing paragraph warning about _FILE_OFFSET_BITS default changes
due to Y2038 is correct but confusing, as it does not explicitly state
why time_t concerns affect _FILE_OFFSET_BITS.

Clarify that _TIME_BITS=64 (needed for Y2038 safety) requires
_FILE_OFFSET_BITS=64, so when systems migrate to 64-bit time_t by
default, _FILE_OFFSET_BITS will also need to default to 64, even for
applications that do not handle large files.

This addresses the confusion noted in the bug report while keeping the
warning in place, as the transitive dependency makes it relevant to
the _FILE_OFFSET_BITS documentation.

Signed-off-by: Shamil Abdulaev <ashamil435@gmail.com>