Add conditional backend infos to docs

doc/help/settings.asciidoc

@@ -1305,7 +1305,6 @@ Default: empty
 [[content.print_element_backgrounds]]
 == content.print_element_backgrounds
 Whether the background color and images are also drawn when the page is printed.
-# FIXME:conf This should be added automatically: # This setting only works with Qt 5.8 or newer when using the QtWebEngine backend.
 
 Valid values:
 
@@ -1314,6 +1313,8 @@ Valid values:
 
 Default: +pass:[true]+
 
+On QtWebEngine, this setting requires Qt 5.8 or newer.
+
 [[content.private_browsing]]
 == content.private_browsing
 Open new windows in private browsing mode which does not record visited pages.
@@ -1329,7 +1330,6 @@ Default: empty
 == content.proxy
 The proxy to use.
 In addition to the listed values, you can use a `socks://...` or `http://...` URL.
-# FIXME:conf This should be added automatically: # This setting only works with Qt 5.8 or newer when using the QtWebEngine # backend.
 
 Valid values:
 
@@ -1338,6 +1338,8 @@ Valid values:
 
 Default: +pass:[system]+
 
+On QtWebEngine, this setting requires Qt 5.8 or newer.
+
 [[content.proxy_dns_requests]]
 == content.proxy_dns_requests
 Whether to send DNS requests over the configured proxy.

qutebrowser/config/configdata.py

@@ -47,7 +47,7 @@ MONOSPACE = (' xos4 Terminus, Terminus, Monospace, '
 
 
 Option = collections.namedtuple('Option', ['name', 'typ', 'default',
-                                           'backends', 'conditional_backends',
+                                           'backends', 'raw_backends',
                                            'description'])
 
 
@@ -154,18 +154,16 @@ def _parse_yaml_backends(name, node):
       -> setting available based on the given conditionals.
 
     Return:
-        A list of backends, and a boolean whether the list is conditional based
-        on the Qt version.
+        A list of backends.
     """
     if node is None:
-        return ([usertypes.Backend.QtWebKit, usertypes.Backend.QtWebEngine],
-                False)
+        return [usertypes.Backend.QtWebKit, usertypes.Backend.QtWebEngine]
     elif node == 'QtWebKit':
-        return ([usertypes.Backend.QtWebKit], False)
+        return [usertypes.Backend.QtWebKit]
     elif node == 'QtWebEngine':
-        return ([usertypes.Backend.QtWebEngine], False)
+        return [usertypes.Backend.QtWebEngine]
     elif isinstance(node, dict):
-        return (_parse_yaml_backends_dict(name, node), True)
+        return _parse_yaml_backends_dict(name, node)
     _raise_invalid_node(name, 'backends', node)
 
 
@@ -188,15 +186,14 @@ def _read_yaml(yaml_data):
             raise ValueError("Invalid keys {} for {}".format(
                 option.keys(), name))
 
-        backends, conditional_backends = _parse_yaml_backends(
-            name, option.get('backend', None))
+        backends = option.get('backend', None)
 
         parsed[name] = Option(
             name=name,
             typ=_parse_yaml_type(name, option['type']),
             default=option['default'],
-            backends=backends,
-            conditional_backends=conditional_backends,
+            backends=_parse_yaml_backends(name, backends),
+            raw_backends=backends if isinstance(backends, dict) else None,
             description=option['desc'])
 
     # Make sure no key shadows another.

qutebrowser/config/configdata.yml

@@ -439,9 +439,6 @@ content.print_element_backgrounds:
     Whether the background color and images are also drawn when the page is
     printed.
 
-    # FIXME:conf This should be added automatically:
-    # This setting only works with Qt 5.8 or newer when using the QtWebEngine backend.
-
 content.private_browsing:
   type: Bool
   default: false
@@ -460,10 +457,6 @@ content.proxy:
     In addition to the listed values, you can use a `socks://...` or `http://...`
     URL.
 
-    # FIXME:conf This should be added automatically:
-    # This setting only works with Qt 5.8 or newer when using the QtWebEngine
-    # backend.
-
 content.proxy_dns_requests:
   default: true
   type: Bool

scripts/dev/src2asciidoc.py

@@ -387,7 +387,16 @@ def _generate_setting_option(f, opt):
         f.write("Default: empty\n")
 
     all_backends = [usertypes.Backend.QtWebKit, usertypes.Backend.QtWebEngine]
-    if opt.backends == all_backends or opt.conditional_backends:
+    if opt.raw_backends is not None:
+        for name, conditional in sorted(opt.raw_backends.items()):
+            if conditional is True:
+                pass
+            elif conditional is False:
+                f.write("\nOn {}, this setting is unavailable.\n".format(name))
+            else:
+                f.write("\nOn {}, this setting requires {} or newer.\n"
+                        .format(name, conditional))
+    elif opt.backends == all_backends:
         pass
     elif opt.backends == [usertypes.Backend.QtWebKit]:
         f.write("\nThis setting is only available with the QtWebKit "