Factored out common venv documentation and added more information about Distribute/pip.

Doc/using/venv-create.inc

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+Creation of :ref:`virtual environments <venv-def>` is done by executing the
+``pyvenv`` script::
+
+    pyvenv /path/to/new/virtual/environment
+
+Running this command creates the target directory (creating any parent
+directories that don't exist already) and places a ``pyvenv.cfg`` file in it
+with a ``home`` key pointing to the Python installation the command was run
+from.  It also creates a ``bin`` (or ``Scripts`` on Windows) subdirectory
+containing a copy of the ``python`` binary (or binaries, in the case of
+Windows).  It also creates an (initially empty) ``lib/pythonX.Y/site-packages``
+subdirectory (on Windows, this is ``Lib\site-packages``).
+
+.. highlight:: none
+
+On Windows, you may have to invoke the ``pyvenv`` script as follows, if you
+don't have the relevant PATH and PATHEXT settings::
+
+    c:\Temp>c:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv
+
+or equivalently::
+
+    c:\Temp>c:\Python33\python -m venv myenv
+
+The command, if run with ``-h``, will show the available options::
+
+    usage: pyvenv [-h] [--system-site-packages] [--symlinks] [--clear]
+                  [--upgrade] ENV_DIR [ENV_DIR ...]
+
+    Creates virtual Python environments in one or more target directories.
+
+    positional arguments:
+      ENV_DIR             A directory to create the environment in.
+
+    optional arguments:
+      -h, --help             show this help message and exit
+      --system-site-packages Give access to the global site-packages dir to the
+                             virtual environment.
+      --symlinks             Try to use symlinks rather than copies, when symlinks
+                             are not the default for the platform.
+      --clear                Delete the environment directory if it already exists.
+                             If not specified and the directory exists, an error is
+                             raised.
+      --upgrade              Upgrade the environment directory to use this version
+                             of Python, assuming Python has been upgraded in-place.
+
+If the target directory already exists an error will be raised, unless
+the ``--clear`` or ``--upgrade`` option was provided.
+
+The created ``pyvenv.cfg`` file also includes the
+``include-system-site-packages`` key, set to ``true`` if ``venv`` is
+run with the ``--system-site-packages`` option, ``false`` otherwise.
+
+Multiple paths can be given to ``pyvenv``, in which case an identical
+virtualenv will be created, according to the given options, at each
+provided path.
+
+Once a venv has been created, it can be "activated" using a script in the
+venv's binary directory. The invocation of the script is platform-specific: on
+a Posix platform, you would typically do::
+
+    $ source <venv>/bin/activate
+
+whereas on Windows, you might do::
+
+    C:\> <venv>/Scripts/activate
+
+if you are using the ``cmd.exe`` shell, or perhaps::
+
+    PS C:\> <venv>/Scripts/Activate.ps1
+
+if you use PowerShell.
+
+You don't specifically *need* to activate an environment; activation just
+prepends the venv's binary directory to your path, so that "python" invokes the
+venv's Python interpreter and you can run installed scripts without having to
+use their full path. However, all scripts installed in a venv should be
+runnable without activating it, and run with the venv's Python automatically.
+
+You can deactivate a venv by typing "deactivate" in your shell. The exact
+mechanism is platform-specific: for example, the Bash activation script defines
+a "deactivate" function, whereas on Windows there are separate scripts called
+``deactivate.bat`` and ``Deactivate.ps1`` which are installed when the venv is
+created.
+