qapi: add 'If:' condition to enum values documentation

Use a common function to generate the "If:..." line.

While at it, get rid of the existing \n\n (no idea why it was
there). Use a line-break in member description, this seems to look
slightly better in the plaintext version.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181213123724.4866-19-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/doc-good.json

@@ -55,7 +55,9 @@
 #
 # @two is undocumented
 ##
-{ 'enum': 'Enum', 'data': [ 'one', 'two' ], 'if': 'defined(IFCOND)' }
+{ 'enum': 'Enum', 'data':
+  [ { 'name': 'one', 'if': 'defined(IFONE)' }, 'two' ],
+  'if': 'defined(IFCOND)' }
 
 ##
 # @Base:

tests/qapi-schema/doc-good.out

@@ -11,6 +11,7 @@ enum QType
 module doc-good.json
 enum Enum
     member one
+        if ['defined(IFONE)']
     member two
     if ['defined(IFCOND)']
 object Base

tests/qapi-schema/doc-good.texi

@@ -84,12 +84,12 @@ Examples:
 @table @asis
 @item @code{one}
 The @emph{one} @{and only@}
+@*@b{If:} @code{defined(IFONE)}
 @item @code{two}
 Not documented
 @end table
 @code{two} is undocumented
 
-
 @b{If:} @code{defined(IFCOND)}
 @end deftp