maint: Update references to ChangeLog*

The files no longer exist, at least not in their previous form,
so references to them need to be reworked to still make sense.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/news-ascii.xsl

@@ -15,7 +15,8 @@
     <xsl:text>
 ==============================================================================
 Older libvirt releases didn't have proper release notes: if you are interested
-in changes between them, you should check out ChangeLog* and docs/news-*.html.
+in changes between them, you should check out docs/news-*.html or the full git
+log (see instructions in ChangeLog).
 </xsl:text>
   </xsl:template>