ref-manual: Added review changes to BBCLASSEXTEND description

Fixes [YOCTO #9909]

Added some technical clarifications to the existing note to be clear
about what is causing the limitations in this case.  Applied some
formatting fixed to for the use of the include word.

(From yocto-docs rev: cbbd960d8665d9fb6b92698c14728d1c0bcc659f)

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

documentation/ref-manual/ref-variables.xml

@@ -1001,19 +1001,21 @@
                         mechanism generates recipe variants by rewriting
                         variable values and applying overrides such as
                         <filename>_class-native</filename>.
-                        For example, to generate a native version of a recipe
+                        For example, to generate a native version of a recipe,
-                        named "foo", a
+                        a
                         <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                         on "foo" is rewritten to a <filename>DEPENDS</filename>
                         on "foo-native".
                         </para>
 
                         <para>
-                        No reparsing, which adds some limitations, of the
+                        Even when using <filename>BBCLASSEXTEND</filename>, the
-                        recipe is done for each variant.
+                        recipe is only parsed once.
-                        For example, it is not possible to "include" a
+                        Parsing once adds some limitations.
-                        different file depending on the variant, since
+                        For example, it is not possible to
-                        "include's" are processed when the recipe is parsed.
+                        include a different file depending on the variant,
+                        since <filename>include</filename> statements are
+                        processed when the recipe is parsed.
                         </para>
                     </note>
                 </para>