Add useful tips to keyboard test routine

doc/keyboard-test.txt

@@ -3,6 +3,15 @@ Test protocol for keyboard handling
 
 Platform specific tests/issues are marked with [].
 
+These tests are primarily about what is sent over the protocol and some
+may be difficult or impossible to test using normal applications. In
+these cases you can turn on debug logging in either the client or the
+server by specifying "-Log *:stderr:100".
+
+We currently have a limitation in Xvnc where it can run out of symbols,
+resulting in nothing being sent to the applications. Just run setxkbmap
+with any layout to reset the symbol table.
+
 Client
 ------
 
@@ -15,6 +24,13 @@ Client
 
 - Dead keys:
 
+  These are not available on a single layout, so you need to switch to
+  test everything. The useful layouts also differ per platform:
+
+    X11: US International for most, Greek for some specific diacretics
+    Win: US International, Czech, Greek Polytonic, Vietnamese
+    OS X: ABC Extended, Greek
+
   - Grave
   - Acute
   - Circumflex
@@ -38,6 +54,9 @@ Client
 
 - Modifiers:
 
+  X11: You can usually toggle Hyper/Super and Compose/Scroll_Lock using
+       XKB options.
+
   - CapsLock, NumLock (sent but ignored by server)
   - Shift, Ctrl
   - Alt, AltGr, Super [Win, X11] (FIXME: AltGr broken on Win)