mdev.txt: explain the meaning of a leading "-"

If a line in mdev.conf starts with "-", parsing will continue even if
a match is found in that line.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

docs/mdev.txt

@@ -60,9 +60,9 @@ or
 For example:
 	hd[a-z][0-9]* 0:3 660
 
-The config file parsing stops at the first matching line.  If no line is
-matched, then the default of 0:0 660 is used.  To set your own default, simply
-create your own total match like so:
+The config file parsing stops at the first matching line unless this line
+starts with "-". If no line is matched, then the default of 0:0 660 is used.
+To set your own default, simply create your own total match like so:
 
 	.* 1:1 777