[stopsource.inplace.mem] Rephrase for brevity

Also avoids an overfull \hbox.

Author: jensmaurer

source/threads.tex

@@ -1290,9 +1290,8 @@
 \begin{itemdescr}
 \pnum
 \returns
-A new associated \tcode{inplace_stop_token} object.
-The \tcode{inplace_stop_token} object's \exposid{stop-source} member
-is equal to \tcode{this}.
+A new associated \tcode{inplace_stop_token} object
+whose \exposid{stop-source} member is equal to \tcode{this}.
 \end{itemdescr}
 
 \begin{itemdecl}