[enum.copy_options] Rephrase restriction on copy_options values.

Fixes #1422.

Author: jensmaurer

source/iostreams.tex

@@ -12193,9 +12193,10 @@
 is a bitmask type~(\ref{bitmask.types}) that specifies bitmask constants used to control the semantics of
 copy operations. The constants are specified in option groups with the meanings listed in Table~\ref{tab:enum.copy_options}.
 Constant \tcode{none} is shown in each option group for purposes of exposition;
-implementations shall provide only a single definition. Calling a
-library function with more than a single constant for an option
-group results in undefined behavior.
+implementations shall provide only a single definition.
+A value of type \tcode{copy_options} that is passed to a function
+described in subclause \ref{filesystems} shall have at most one
+element set from each option group.
 
 \begin{floattable}
 {Enum class \tcode{copy_options}}{tab:enum.copy_options}