qapi: add 'If:' condition to struct members documentation

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181213123724.4866-20-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

@@ -72,7 +72,8 @@
 #
 # Another paragraph (but no @var: line)
 ##
-{ 'struct': 'Variant1', 'data': { 'var1': 'str' } }
+{ 'struct': 'Variant1',
+  'data': { 'var1': { 'type': 'str', 'if': 'defined(IFSTR)' } } }
 
 ##
 # @Variant2:

tests/qapi-schema/doc-good.out

@@ -18,6 +18,7 @@ object Base
     member base1: Enum optional=False
 object Variant1
     member var1: str optional=False
+        if ['defined(IFSTR)']
 object Variant2
 object Object
     base Base

tests/qapi-schema/doc-good.texi

@@ -119,6 +119,7 @@ Another paragraph (but no @code{var}: line)
 @table @asis
 @item @code{var1: string}
 Not documented
+@*@b{If:} @code{defined(IFSTR)}
 @end table
 
 @end deftp