[fs.op.remove] Clarify "Returns" element (#7387)

To avoid confusion as in microsoft/STL#5088.

Author: CaseyCarter

source/iostreams.tex

@@ -18425,9 +18425,10 @@
 
 \pnum
 \returns
-\tcode{false} if \tcode{p} did not exist,
-  otherwise \tcode{true}. The signature with argument \tcode{ec}
-  returns \tcode{false} if an error occurs.
+\tcode{true} if a file \tcode{p} has been removed and \tcode{false} otherwise.
+\begin{note}
+Absence of a file \tcode{p} is not an error.
+\end{note}
 
 \pnum
 \throws