Include the --prompt flag in venv's help output (GH-10711)

Document usage of the existing `--prompt` option in the command line help.
(cherry picked from commit 3208880)

Co-authored-by: Holger Frey <[email protected]>

Author: miss-islington

Doc/using/venv-create.inc

@@ -35,7 +35,7 @@ your :ref:`Python installation <using-on-windows>`::
 The command, if run with ``-h``, will show the available options::
 
     usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear]
-                [--upgrade] [--without-pip]
+                [--upgrade] [--without-pip] [--prompt PROMPT]
                 ENV_DIR [ENV_DIR ...]
 
     Creates virtual Python environments in one or more target directories.
@@ -58,6 +58,8 @@ The command, if run with ``-h``, will show the available options::
                             of Python, assuming Python has been upgraded in-place.
       --without-pip         Skips installing or upgrading pip in the virtual
                             environment (pip is bootstrapped by default)
+      --prompt PROMPT       Provides an alternative prompt prefix for this
+                            environment.
 
     Once an environment has been created, you may wish to activate it, e.g. by
     sourcing an activate script in its bin directory.