[expr.await] Define 'suspend' as a term of power for coroutines

Many parts of the standard refer to whether a coroutine is suspended as if
it were a term of power, but there is no \defn that defines it as such.
Editorial review suggests this commit provides the best location to make
the definition."

Author: AlisdairM

source/expressions.tex

@@ -5437,7 +5437,7 @@
 \indextext{\idxcode{co_await}}%
 
 \pnum
-The \keyword{co_await} expression is used to suspend evaluation of a
+The \keyword{co_await} expression is used to \defnx{suspend}{suspend a coroutine} evaluation of a
 coroutine\iref{dcl.fct.def.coroutine} while awaiting completion of
 the computation represented by the operand expression.
 Suspending the evaluation of a coroutine