system_data_types.7: srcfix: add comment noting time_t difference in POSIX.1-2001

system_data_types.7: srcfix: add comment noting time_t difference in POSIX.1-2001

Paul Eggert commented on a patch that proposed to note the
POSIX.2001 details:

    No actual POSIXish implementation ever made it a
    real-floating type, though, and that point should be made
    lest some conscientious programmer worry about a nonexistent
    porting issue.

We opted to drop the patch, but in case someone else points out
this POSIX.1-2001 difference in the future, let's leave a comment
in the page source.

Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>