Mention that source order only matter per version

Closes: #617445

doc/sources.list.5.xml

@@ -36,7 +36,7 @@
       designed to support any number of active sources and a variety of source
       media. The files list one source per line (one line style) or contain multiline
       stanzas defining one or more sources per stanza (deb822 style), with the
-      most preferred source listed first. The information available from the
+      most preferred source listed first (in case a single version is available from more than one source). The information available from the
       configured sources is acquired by <command>apt-get update</command> (or
       by an equivalent command from another APT front-end).
    </para>