update docs to explain why setting elements on mstyle_new_default is a

terrible idea.

doc/Styles.txt

@@ -21,7 +21,8 @@ adopted. This document breaks roughly into two parts. First the 'user'
 
 	In future named styles will be possible, also to get the default style
 	use mstyle_new_default (). NB. the name 'mstyle' is standard for this
-	structure's instances.
+	structure's instances. NB. do _not_ set any of the fields on the default
+	style.
 
 	b) Set elements:
 

doc/developer/Styles.txt

@@ -21,7 +21,8 @@ adopted. This document breaks roughly into two parts. First the 'user'
 
 	In future named styles will be possible, also to get the default style
 	use mstyle_new_default (). NB. the name 'mstyle' is standard for this
-	structure's instances.
+	structure's instances. NB. do _not_ set any of the fields on the default
+	style.
 
 	b) Set elements:
 

src/mstyle.c

@@ -486,6 +486,15 @@ mstyle_new_name (const gchar *name)
 
 static MStyle *default_mstyle = NULL;
 
+/**
+ * mstyle_new_default:
+ * 
+ * Return the default style,
+ * this should _never_ _ever_ have any of its elements
+ * set.
+ * 
+ * Return value: the default style.
+ **/
 MStyle *
 mstyle_new_default (void)
 {