[expr.prim.req] Use grammar terms rather than undefined phrases when

referring to kinds of *requirement*.

Author: zygoloid

source/expressions.tex

@@ -1644,7 +1644,8 @@
 \end{bnf}
 
 \pnum
-A simple requirement asserts the validity of an expression.
+A \grammarterm{simple-requirement} asserts
+the validity of an \grammarterm{expression}.
 \begin{note}
 The enclosing \grammarterm{requires-expression} will evaluate to \tcode{false}
 if substitution of template arguments into the \grammarterm{expression} fails.
@@ -1668,7 +1669,7 @@
 \end{bnf}
 
 \pnum
-A type requirement asserts the validity of a type.
+A \grammarterm{type-requirement} asserts the validity of a type.
 \begin{note}
 The enclosing \grammarterm{requires-expression} will evaluate to \tcode{false}
 if substitution of template arguments fails.
@@ -1687,7 +1688,7 @@
 \end{example}
 
 \pnum
-A type requirement that names a class template specialization
+A \grammarterm{type-requirement} that names a class template specialization
 does not require that type to be complete~(\ref{basic.types}).
 
 \rSec3[expr.prim.req.compound]{Compound requirements}