[flat.map.cons] Clarify that the state before the move is meant.

Author: tkoeppe

source/containers.tex

@@ -14836,7 +14836,7 @@
 \complexity
 Linear in $N$ if the container arguments are already sorted with respect
 to \tcode{value_comp()} and otherwise $N \log N$,
-where $N$ is \tcode{key_cont.size()}.
+where $N$ is the value of \tcode{key_cont.size()} before this call.
 \end{itemdescr}
 
 \indexlibraryctor{flat_map}%