[range.nonprop.cache] Clarify emplace-deref for prvalues. (#4732)

A new note explains that `emplace-deref` requires implementations to
avoid materialization of the result of `*i` before the initialization.
This is implied by the normatively expressed requirement in terms of
an invented initialization of a variable, but is easily overlooked.

Author: timsong-cpp

source/ranges.tex

@@ -3216,6 +3216,10 @@
  \mandates
 The declaration \tcode{T t(*i);} is well-formed
 for some invented variable \tcode{t}.
+\begin{note}
+If \tcode{*i} is a prvalue of type \cv{} \tcode{T},
+there is no requirement that it is movable\iref{dcl.init.general}.
+\end{note}
 
  \effects
 Calls \tcode{reset()}.