Add Wall of Browser Bugs entry for #18859

See https://bugs.webkit.org/show_bug.cgi?id=153056
Refs #18859
[skip sauce]

docs/_data/browser-bugs.yml

@@ -300,6 +300,16 @@
   origin: >
     Bootstrap#16028
 
+-
+  browser: >
+    Safari (iOS)
+  summary: >
+    `position:fixed` is incorrectly positioned when tab bar is visible on iPhone 6S+ Safari
+  upstream_bug: >
+    WebKit#153056
+  origin: >
+    Bootstrap#18859
+
 -
   browser: >
     Safari (iPad Pro)