doc: clarify that keys can be overridden

doc/hgrc.5.txt

@@ -90,7 +90,8 Syntax
 ------
 
 A configuration file consists of sections, led by a ``[section]`` header
-and followed by ``name = value`` entries::
+and followed by ``name = value`` entries (sometimes called
+``configuration keys``)::
 
     [spam]
     eggs=ham
@@ -102,6 +103,16 they are treated as continuations of tha
 removed from values. Empty lines are skipped. Lines beginning with
 ``#`` or ``;`` are ignored and may be used to provide comments.
 
+Configuration keys can be set multiple times, in which case mercurial
+will use the value that was configured last. As an example::
+
+    [spam]
+    eggs=large
+    ham=serrano
+    eggs=small
+
+This would set the configuration key named ``eggs`` to ``small``.
+
 A line of the form ``%include file`` will include ``file`` into the
 current configuration file. The inclusion is recursive, which means
 that included files can include other files. Filenames are relative to