[flat.{,multi}set.ctor] Clarify that the state before the move is meant.

Author: tkoeppe

source/containers.tex

@@ -16465,7 +16465,7 @@
 \pnum
 \complexity
 Linear in $N$ if \tcode{cont} is sorted with respect to \tcode{compare} and
-otherwise $N \log N$, where $N$ is \tcode{cont.size()}.
+otherwise $N \log N$, where $N$ is the value of \tcode{cont.size()} before this call.
 \end{itemdescr}
 
 \indexlibraryctor{flat_set}%
@@ -17069,7 +17069,7 @@
 \pnum
 \complexity
 Linear in $N$ if \tcode{cont} is sorted with respect to \exposid{compare} and
-otherwise $N \log N$, where $N$ is \tcode{cont.size()}.
+otherwise $N \log N$, where $N$ is the value of \tcode{cont.size()} before this call.
 \end{itemdescr}
 
 \indexlibraryctor{flat_multiset}%