fixup! Update KEEP on Explicit backing fields feature

Author: merfemor

Diff for: proposals/explicit-backing-fields.md

@@ -327,6 +327,7 @@ Property with explicit backing field:
 * must be initialized right away. It can't be initialized from `init` block
   (otherwise we can't differentiate whether initializer expected here or not)
 * can't be `var` if it has property initializer (to prevent ambiguity of what should be mutable here)
+* can't be extension property
 
 Neither property with explicit backing field:
 * nor its accessors can be `inline`