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.

man/tmpfiles.d.xml

@@ -579,6 +579,13 @@ r! /tmp/.X[0-9]*-lock</programlisting>
       <literal>~</literal>, the clean-up is only applied to files and
       directories one level inside the directory specified, but not
       the files and directories immediately inside it.</para>
+
+      <para>The age of a file system entry is determined from its last
+      modification timestamp (mtime), its last access timestamp (atime),
+      and (except for directories) its last status change timestamp
+      (ctime). Any of these three (or two) values will prevent cleanup
+      if it is more recent than the current time minus the age
+      field.</para>
     </refsect2>
 
     <refsect2>