[thread.barrier.class] Rename constructor parameter from phase_count

to `expected`.

The parameter is not a phase count, and is referred to by other
normative wording as `expected`; also, `expected` is the name we use for
the same parameter in the constructor of `latch`.

Author: zygoloid

source/threads.tex

@@ -3980,7 +3980,7 @@
   public:
     using arrival_token = @\seebelow@;
 
-    constexpr explicit barrier(ptrdiff_t phase_count,
+    constexpr explicit barrier(ptrdiff_t expected,
                                CompletionFunction f = CompletionFunction());
     ~barrier();
 
@@ -4073,23 +4073,23 @@
 
 \indexlibrary{\idxcode{barrier}!constructor}
 \begin{itemdecl}
-constexpr explicit barrier(ptrdiff_t phase_count,
+constexpr explicit barrier(ptrdiff_t expected,
                            CompletionFunction f = CompletionFunction());
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{phase_count >= 0} is \tcode{true}.
+\tcode{expected >= 0} is \tcode{true}.
 
 \pnum
 \effects
 Sets both the initial expected count for each barrier phase and
-the current expected count for the first phase to \tcode{phase_count}.
+the current expected count for the first phase to \tcode{expected}.
 Initializes \tcode{completion} with \tcode{std::move(f)}.
 Starts the first phase.
 \begin{note}
-If \tcode{phase_count} is 0 this object can only be destroyed.
+If \tcode{expected} is 0 this object can only be destroyed.
 \end{note}
 
 \pnum