Add a note that constexpr functions are never key functions.

Author: zygoloid

abi.html

@@ -6027,6 +6027,8 @@ <h4><a href="#vague-vtable"> 5.2.3 Virtual Tables</a></h4>
 Note that if the key function is not declared inline in the class definition,
 but its definition later is always declared inline,
 it will be emitted in every object containing the definition.</i>
+A constexpr or consteval function is always declared constexpr or consteval on
+its first declaration, and is implicitly inline, so is never a key function.
 
 <p>
 <img src=warning.gif alt="<b>NOTE</b>:">