[dcl.constexpr] Consistently describe rules for validity of constexpr

function definitions as "requirements" so that the cross-references
to this definition use the same term as the definition itself.

Author: zygoloid

source/declarations.tex

@@ -790,7 +790,7 @@
 \indextext{specifier!\idxcode{constexpr}!function}
 \indextext{constexpr function}
 The definition of a \tcode{constexpr} function shall satisfy the following
-constraints:
+requirements:
 
 \begin{itemize}
 \item
@@ -856,7 +856,7 @@
 \pnum
 \indextext{specifier!\idxcode{constexpr}!constructor}%
 The definition of a \tcode{constexpr} constructor shall satisfy the
-following constraints:
+following requirements:
 
 \begin{itemize}
 \item
@@ -874,12 +874,12 @@
 \end{itemize}
 
 In addition, either its \grammarterm{function-body} shall be
-\tcode{= delete}, or it shall satisfy the following constraints:
+\tcode{= delete}, or it shall satisfy the following requirements:
 
 \begin{itemize}
 \item
 either its \grammarterm{function-body} shall be \tcode{= default}, or the \grammarterm{compound-statement} of its \grammarterm{function-body}
-shall satisfy the constraints for a \grammarterm{function-body} of a
+shall satisfy the requirements for a \grammarterm{function-body} of a
 \tcode{constexpr} function;
 
 \item