doc: improves merge-tools example with an overwrite of existing config

We highlight the fact that a user hgrc can overwrite the system wide
preconfiguration. As other benefit we show priority value other than 1 and shows
vimdiff configuration is usually directly available. This is valuable as vimdiff
is (surprisingly) a common request from user.

mercurial/help/config.txt

@@ -821,6 +821,9 @@ Example ``~/.hgrc``::
   # Give higher priority
   kdiff3.priority = 1
 
+  # Changing the priority of preconfigured tool
+  vimdiff.priority = 0
+
   # Define new tool
   myHtmlTool.args = -m $local $other $base $output
   myHtmlTool.regkey = Software\FooSoftware\HtmlMerge