man: document meaning of age in tmpfiles.d (#8092)

man: document meaning of age in tmpfiles.d (#8092)

This documents how the age of a file is determined, which previously was
only alluded to in other parts of the documentation. Fixes #8091.

The phrasings of “last modification timestamp” etc. are taken from
man:inode(7) (as of man-pages 4.14). The debug messages in tmpfiles.c
use different messages (“modify time”), which according to a code
comment follow man:stat(1); however, my copy of that manpage (from GNU
coreutils 8.29) documents %y as “time of last data modification”
instead.