Add comment about implementation to 4164 P/R

Author: jwakely

xml/issue4164.xml

@@ -53,6 +53,11 @@ A "no effects" guarantee is already given for `push_front` and `emplace_front`
 in <sref ref="[container.reqmts]"/> p66, although that doesn't say anything
 about iterator invalidation so we might want to add that to
 <sref ref="[forward.list.modifiers]"/> too.
+For the functions that insert a single element, it's trivial to not modify
+the list if allocating a new node of constructing the element throws.
+The strong exception safety guarantee for the multi-element insertion functions
+is easily achieved by inserting into a temporary `forward_list` first,
+then using `splice_after` which is non-throwing.
 </p>
 
 </discussion>