Improve text for "keep above"/"keep below" buttons

It may not be totally clear what exactly they're being kept above.

CCBUG: 435292

src/kcmkwin/kwindecoration/declarative-plugin/buttonsmodel.cpp

@@ -67,9 +67,9 @@ static QString buttonToName(DecorationButtonType type)
         case DecorationButtonType::Shade:
             return i18n("Shade");
         case DecorationButtonType::KeepBelow:
-            return i18n("Keep below");
+            return i18n("Keep below other windows");
         case DecorationButtonType::KeepAbove:
-            return i18n("Keep above");
+            return i18n("Keep above other windows");
         default:
             return QString();
     }

src/kcmkwin/kwinrules/rulesmodel.cpp

@@ -555,12 +555,12 @@ void RulesModel::populateRuleList()
     // Arrangement & Access
     addRule(new RuleItem(QLatin1String("above"),
                          RulePolicy::SetRule, RuleItem::Boolean,
-                         i18n("Keep above"), i18n("Arrangement & Access"),
+                         i18n("Keep above other windows"), i18n("Arrangement & Access"),
                          QIcon::fromTheme("window-keep-above")));
 
     addRule(new RuleItem(QLatin1String("below"),
                          RulePolicy::SetRule, RuleItem::Boolean,
-                         i18n("Keep below"), i18n("Arrangement & Access"),
+                         i18n("Keep below other windows"), i18n("Arrangement & Access"),
                          QIcon::fromTheme("window-keep-below")));
 
     addRule(new RuleItem(QLatin1String("skiptaskbar"),