DeveloperGuide: use labeled list for glossary

The glossary uses literal blocks for formatting definitions, which looks
strange because literal blocks are usually used for preformatted text
such as source code.

Instead, let's use a labeled list, which has a formatting that is more
in line with a glossary definition list.

Convert the paragraph anchors into inline anchors[1], so that the items
are joined into one single labeled list with two items, rather than
being chopped into two separate labeled lists with one item each.

[1] See http://asciidoctor.org/docs/user-manual/#anchordef for the
    difference between a paragraph anchor and an inline anchor.

docs/DeveloperGuide.adoc

@@ -781,19 +781,11 @@ Use case resumes at step 2.
 [appendix]
 == Glossary
 
-[[mainstream-os]]
-Mainstream OS
-
-....
+[[mainstream-os]] Mainstream OS::
 Windows, Linux, Unix, OS-X
-....
-
-[[private-contact-detail]]
-Private contact detail
 
-....
+[[private-contact-detail]] Private contact detail::
 A contact detail that is not meant to be shared with others
-....
 
 [appendix]
 == Product Survey