[algorithms.parallel.defns] Rename to 'Preamble' (#3350)

and avoid hanging paragraph in [algorithms.parallel]
by moving it here.

Fixes NB JP 314 (C++20 CD)

Author: jensmaurer

source/algorithms.tex

@@ -270,11 +270,11 @@
 
 \rSec1[algorithms.parallel]{Parallel algorithms}
 
+\rSec2[algorithms.parallel.defns]{Preamble}
 \pnum
-This subclause describes components that \Cpp{} programs may use
+Subclause \ref{algorithms.parallel} describes components that \Cpp{} programs may use
 to perform operations on containers and other sequences in parallel.
 
-\rSec2[algorithms.parallel.defns]{Terms and definitions}
 \pnum
 A \defn{parallel algorithm} is a function template listed in this document
 with a template parameter named \tcode{ExecutionPolicy}.