Update std.range.* docs.

The previous docs were simply dissected from the original std.range docs, and
the text in each module is truncated and awkward to read. The main package.d
docs were also missing links to the submodules. So, update the docs to reflect
the current state of things.

std/range/interfaces.d

@@ -1,8 +1,11 @@
 /**
-These _range-construction tools are implemented using templates; but sometimes
-an object-based interface for ranges is needed. For this purpose, this module
+This module is a submodule of $(LINK2 std_range_package.html, std.range).
+
+The main $(D std.range) module provides template-based tools for working with
+ranges, but sometimes an object-based interface for ranges is needed, such as
+when runtime polymorphism is required. For this purpose, this submodule
 provides a number of object and $(D interface) definitions that can be used to
-wrap around _range objects created by the above templates.
+wrap around _range objects created by the $(D std.range) templates.
 
 $(BOOKTABLE ,
     $(TR $(TD $(D $(LREF InputRange)))

std/range/package.d

@@ -14,8 +14,21 @@ motivation behind them, see Andrei Alexandrescu's article
 $(LINK2 http://www.informit.com/articles/printerfriendly.aspx?p=1407357&rll=1,
 $(I On Iteration)).
 
-A rich set of _range creation and composition templates are provided that let
-you construct new ranges out of existing ranges:
+Submodules:
+
+This module has a few submodules:
+
+The $(LINK2 std_range_primitives.html, $(D std._range.primitives)) submodule
+provides basic _range functionality. It defines several templates for testing
+whether a given object is a _range, what kind of _range it is, and provides
+some common _range operations.
+
+The $(LINK2 std_range_interfaces.html, $(D std._range.interfaces)) submodule
+provides object-based interfaces for working with ranges via runtime
+polymorphism.
+
+The remainder of this module provides a rich set of _range creation and
+composition templates that let you construct new ranges out of existing ranges:
 
 $(BOOKTABLE ,
     $(TR $(TD $(D $(LREF retro)))

std/range/primitives.d

@@ -1,6 +1,9 @@
 /**
-This module defines several templates for testing whether a given object is a
-_range, and what kind of _range it is:
+This module is a submodule of $(LINK2 std_range_package.html, std.range).
+
+It provides basic range functionality by defining several templates for testing
+whether a given object is a _range, and what kind of _range it is:
+
 $(BOOKTABLE ,
     $(TR $(TD $(D $(LREF isInputRange)))
         $(TD Tests if something is an $(I input _range), defined to be
@@ -30,7 +33,7 @@ $(BOOKTABLE ,
     ))
 )
 
-A number of templates are provided that test for various _range capabilities:
+It also provides number of templates that test for various _range capabilities:
 
 $(BOOKTABLE ,
     $(TR $(TD $(D $(LREF hasMobileElements)))
@@ -68,8 +71,7 @@ $(BOOKTABLE ,
     ))
 )
 
-Finally, this module also defines some convenience functions for
-manipulating ranges:
+Finally, it includes some convenience functions for manipulating ranges:
 
 $(BOOKTABLE ,
     $(TR $(TD $(D $(LREF popFrontN)))