[stmt.block] Rephrase introductory sentence.

Author: jensmaurer

source/statements.tex

@@ -192,13 +192,11 @@
 \end{note}
 
 \rSec1[stmt.block]{Compound statement or block}%
-\indextext{statement!compound}%
 \indextext{\idxcode{\{\}}!block statement}%
 
 \pnum
-So that several statements can be used where one is expected, the
-compound statement (also, and equivalently, called ``block'') is
-provided.
+A \defnadj{compound}{statement} (also known as a block) groups a
+sequence of statements into a single statement.
 
 \begin{bnf}
 \nontermdef{compound-statement}\br