[inplace.vector.modifiers] Ensure correct type of returned iterator

Author: eisenwave

source/containers.tex

@@ -11256,9 +11256,10 @@
 
 \pnum
 \returns
-An iterator pointing to the first element of \tcode{rg}
+The first iterator in the range
+\countedrange{ranges::begin(rg)}{n}
 that was not inserted into \tcode{*this},
-or \tcode{ranges::end(rg)} if no such element exists.
+where \tcode{n} is the number of elements in \tcode{rg}.
 
 \pnum
 \complexity