Document the create_gaps option

and sets the default value to false, to keep coherent with the code.

data/default_config.cfg

@@ -342,7 +342,7 @@ theme =
 # Whether to create gaps when moving or closing a tab
 # (a gap means that the number of your tabs does not depend of the previous tabs
 # but only of the creation order)
-create_gaps = true
+create_gaps = false
 
 # If true, a vertical list of tabs, with their name, is displayed on the left of
 # the screen.

doc/source/configuration.rst

@@ -664,6 +664,14 @@ or the way messages are displayed.
 
         Whether or not to display a timestamp before each message.
 
+    create_gaps
+
+        **Default:** ``false``
+
+        Create gaps when moving a tab or closing it. Enabling this option
+        will help you keep the tabs at the same place during the execution of
+        poezio. (gaps are not created when the closed tab is the last one)
+
     popup_time
 
         **Default value:** ``4``