[fs.op.copy], [fs.op.copy_file] Rephrase requirements on copy_options.

Use bitmask 'element' phrasing for restrictions on copy_options.

Fixes #1445.

Author: jensmaurer

source/iostreams.tex

@@ -12416,9 +12416,7 @@
 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:fs.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.
 
 \begin{floattable}
 {Enum class \tcode{copy_options}}{tab:fs.enum.copy_options}
@@ -13839,8 +13837,8 @@
 
 \begin{itemdescr}
 \pnum
-\requires At most one constant from each option group (\ref{fs.enum.copy.opts})
-  is present in \tcode{options}.
+\requires At most one element from each option group (\ref{fs.enum.copy.opts})
+  is set in \tcode{options}.
 
 \pnum
 \effects
@@ -14000,8 +13998,8 @@
 
 \begin{itemdescr}
 \pnum
-\requires At most one constant from each \tcode{copy_options}
-  option group (\ref{fs.enum.copy.opts}) is present
+\requires At most one element from each
+  option group (\ref{fs.enum.copy.opts}) is set
   in \tcode{options}.
 
 \pnum