Help: describe the command pipeline of execute_process more explicitly

When reading the doc page on execute_process, I was mislead by the
words "in parallel". They convinced at once that the processes
are run independently of each other, so that I did not even bother
to read further. The rewording, and not least the insertion of a
paragraph break, should prevent such oversight.

Help/command/execute_process.rst

@@ -22,7 +22,9 @@ Execute one or more child processes.
                   [ERROR_STRIP_TRAILING_WHITESPACE]
                   [ENCODING <name>])
 
-Runs the given sequence of one or more commands in parallel with the standard
+Runs the given sequence of one or more commands.
+
+Commands are executed concurrently as a pipeline, with the standard
 output of each process piped to the standard input of the next.
 A single standard error pipe is used for all processes.