user-manual: Use def_ instead of ref_ for glossary references.

I'd like to start using references to the glossary in the user manual.
The "ref_" prefix for these references seems a little generic; so
replace with "def_".

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/sort_glossary.pl

@@ -50,8 +50,8 @@ ($)
 @keys=sort {uc($a) cmp uc($b)} keys %terms;
 $pattern='(\b(?<!link:git-)'.join('\b|\b(?<!-)',reverse @keys).'\b)';
 foreach $key (@keys) {
-	$terms{$key}=~s/$pattern/sprintf "<<ref_".no_spaces($1).",$1>>";/eg;
-	print '[[ref_'.no_spaces($key).']]'.$key."::\n"
+	$terms{$key}=~s/$pattern/sprintf "<<def_".no_spaces($1).",$1>>";/eg;
+	print '[[def_'.no_spaces($key).']]'.$key."::\n"
 		.format_tab_80($terms{$key})."\n";
 }