move explanation of ;; to commands documentation

The note was out of place in keys.conf as it's not about keyinput. Since
we have a top-level documentation for commands now, let's keep the info
in once place. People will look there anyway for the documentation of
commands they want to bind.

qutebrowser/commands/__init__.py

@@ -28,4 +28,6 @@ For command arguments, there are also some variables you can use:
 - `{url:pretty}` expands to the URL in decoded format
 - `{clipboard}` expands to the clipboard contents
 - `{primary}` expands to the primary selection contents
+
+It is possible to run or bind multiple commands by separating them with `;;`.
 """

qutebrowser/config/configdata.py

@@ -1401,8 +1401,7 @@ KEY_FIRST_COMMENT = """
 #
 # For simple keys (no `<>`-signs), a capital letter means the key is pressed
 # with Shift. For special keys (with `<>`-signs), you need to explicitly add
-# `Shift-` to match a key pressed with shift.  You can bind multiple commands
-# by separating them with `;;`.
+# `Shift-` to match a key pressed with shift.
 #
 # Note that default keybindings are always bound, and need to be explicitly
 # unbound if you wish to remove them: