[fs.op.remove] clarify Returns element

To avoid confusion as in microsoft/STL#5088.

Author: CaseyCarter

source/iostreams.tex

@@ -18425,9 +18425,11 @@
 
 \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.
+The signature with argument \tcode{ec}
+returns \tcode{false} if an error occurs.
+If no error occurs,
+returns \tcode{true} if \tcode{p} did exist and has been removed,
+and \tcode{false} if there is no file \tcode{p}.
 
 \pnum
 \throws