Add Wall of Browser Bugs entry for #16814

Closes #16814
Refs http://wkbug.com/147284
Refs http://openradar.appspot.com/21993128
[skip sauce]

docs/_data/browser-bugs.yml

@@ -178,6 +178,16 @@
   origin: >
     Bootstrap#16372
 
+-
+  browser: >
+    Safari
+  summary: >
+    Incorrect placement of `position: fixed` element when it's a child of a `position: relative; left: X%;` element.
+  upstream_bug: >
+    WebKit#147284, Safari#21993128
+  origin: >
+    Bootstrap#16814
+
 -
   browser: >
     Safari (OS X)