Add documenation about the two modes

doc/scala.txt

@@ -7,8 +7,20 @@ Syntax highlighting and helper functions for the scala language.
 COMMANDS                                        *scala-commands*
 
                                                 *:SortScalaImports*
-:SortScalaImports       Groups the imports a the top of the file and sorts
-                        them alphabetically.
+:SortScalaImports       There are two modes in which this command can operate.
+                        By default it walks all import groups at the top of
+                        the Scala file and orders their lines alphabetically.
+                        A group is a series of lines starting with the
+                        import keyword separated by one or more blank lines.
+
+                        The second, more advanced mode, can be activated by
+                        setting
+
+                          let g:scala_sort_across_groups=1
+
+                        This makes this command include all imports in the
+                        sorting regardless of blank lines in between them and
+                        puts them in three predefined groups instead.
                         The three groups in which the imports can fall are:
 
                           1. Scala and Java core
@@ -24,7 +36,7 @@ COMMANDS                                        *scala-commands*
 
                           g:scala_first_party_namespaces
 
-                        For example in a standard Play! app this would be
+                        For example in a standard Play app this would be
                         set to
                           g:scala_first_party_namespaces='\(controllers\|views\|models\)'