Update documentation for the Natlink and DNS engine back-end

Re: #337, #341.

- Add info on the supported Natlink versions and required Python
  version.
- Include "C:\NatLink\NatLink\MacroSystem" as a common location for
  Natlink's MacroSystem directory.
- Mention Natlink's optional user directory.

documentation/natlink_engine.txt

@@ -6,16 +6,23 @@ Natlink and DNS engine back-end
 Dragonfly uses Natlink to communicate with Dragon.  The latest versions of
 Natlink are available from `SourceForge
 <https://sourceforge.net/projects/natlink/files/natlink/natlinktest4.1/>`__.
+All versions in the 4.X range should work with Dragonfly.
 
 Dragonfly and Natlink support Dragon versions up to 15 (latest). The
 *Individual* editions of *Dragon* are recommended, although other editions
 such as *Home* will also work.
 
+Python version 2.7 (32-bit) is required to use this engine back-end, at
+least for the moment.  Support for this version is not maintained for the
+other engine back-ends and will be **dropped completely** in the first
+*MAJOR* release following stable Natlink support for Python 3.
+
 Once Natlink is up and running, Dragonfly command-modules can be
 treated as any other Natlink macro files.  Natlink automatically
-loads macro files from a predefined directory.  Common locations
-are:
+loads macro files from a predefined directory **or** from the
+optional user directory.  Common locations are:
 
+ * ``C:\NatLink\NatLink\MacroSystem``
  * ``C:\Program Files\NatLink\MacroSystem``
  * ``My Documents\Natlink``