ref-manual, dev-manual: Clarification of "native" and "sdknative"

Fixes [YOCTO #8620]

I went through and made some judgement calls on the use of
"native" and "sdknative".  I tried to make sure that the reader
understood the real meaning of these terms.

(From yocto-docs rev: d711e8c6dfb32a4ad79e9d11dbf44fbc759d0245)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-start.xml

@@ -341,14 +341,17 @@
     </para>
 
     <para>
-        Using a pre-built binary is ideal for developing software applications to run on your
-        target hardware.
-        To do this, you need to be able to access the appropriate cross-toolchain tarball for
-        the architecture on which you are developing.
-        If you are using an SDK type image, the image ships with the complete toolchain native to
-        the architecture.
-        If you are not using an SDK type image, you need to separately download and
-        install the stand-alone Yocto Project cross-toolchain tarball.
+        Using a pre-built binary is ideal for developing software
+        applications to run on your target hardware.
+        To do this, you need to be able to access the appropriate
+        cross-toolchain tarball for the architecture on which you are
+        developing.
+        If you are using an SDK type image, the image ships with the complete
+        toolchain native to the architecture (i.e. a toolchain designed to
+        run on the
+        <ulink url='&YOCTO_DOCS_REF_URL;#var-SDKMACHINE'><filename>SDKMACHINE</filename></ulink>).
+        If you are not using an SDK type image, you need to separately download
+        and install the stand-alone Yocto Project cross-toolchain tarball.
     </para>
 
     <para>