[cpp.pre] Move paragraph introducing preprocessor to first

The paragraph with no normative text that outlines the broad capabilities
of the preprocessor has slippee further down this clause as new text is added.
The most appropriate place for introductory text is the first sentence of the
introductory clause, so moved accordingly.

Author: AlisdairM

source/preprocessor.tex

@@ -173,6 +173,17 @@
     \descr{the new-line character}
 \end{bnf}
 
+\pnum
+The implementation can
+process and skip sections of source files conditionally,
+include other source files,
+import macros from header units,
+and replace macros.
+These capabilities are called
+\defn{preprocessing},
+because conceptually they occur
+before translation of the resulting translation unit.
+
 \pnum
 A \defn{preprocessing directive} consists of a sequence of preprocessing tokens
 that satisfies the following constraints:
@@ -304,17 +315,6 @@
 or possibly other whitespace characters
 in translation phase 3).
 
-\pnum
-The implementation can
-process and skip sections of source files conditionally,
-include other source files,
-import macros from header units,
-and replace macros.
-These capabilities are called
-\defn{preprocessing},
-because conceptually they occur
-before translation of the resulting translation unit.
-
 \pnum
 The preprocessing tokens within a preprocessing directive
 are not subject to macro expansion unless otherwise stated.