Python roles: misc doc updates

Some minor formatting things I noticed when reading through the page
of role documentation.

Change-Id: I2f94102badd83d82073df2dfb1e36a524c7b0686

roles/ensure-pip/README.rst

@@ -55,7 +55,7 @@ qualified name (``pip2`` or ``pip3``) to avoid confusion.
 .. zuul:rolevar:: ensure_pip_virtualenv_cmd
 
    This variable will be set to a command appropriate for general
-   usage with the `pip` module `virtualenv_command` argument on the
-   host.  On Python 3 hosts this will be the inbuilt `venv` module, on
-   Python 2 hosts the `virtualenv` package will be installed (this is
+   usage with the ``pip`` module ``virtualenv_command`` argument on the
+   host.  On Python 3 hosts this will be the inbuilt ``venv`` module, on
+   Python 2 hosts the ``virtualenv`` package will be installed (this is
    avoided on Python 3 hosts as an unnecessary dependency).

roles/ensure-python/README.rst

@@ -10,7 +10,7 @@ Ensure specified python interpreter and development files are installed
   Optional version of python interpreter to install, such as ``3.7``.
 
 .. zuul:rolevar:: python_use_pyenv
-   :default: false
+   :default: False
 
-  Whether to optionally use pyenv to install python instead of distro
-  packages.
+   Whether to optionally use pyenv to install python instead of distro
+   packages.

roles/ensure-sphinx/README.rst

@@ -12,13 +12,13 @@ All pip installs are done with a provided constraints file, if given.
    Optional path to a pip constraints file for installing python libraries.
 
 .. zuul:rolevar:: doc_building_packages
-   :default: ``list``
+   :default: []
 
    List of python packages to install for building docs. The default
    package list is based on the python version in use.
 
 .. zuul:rolevar:: doc_building_extra_packages
-   :default: ``list``
+   :default: []
 
    List of python additional packages to install for building docs.
    By default this list is empty.

roles/ensure-tox/README.rst

@@ -6,7 +6,7 @@ install directory (i.e., ``pip install --user``).
 **Role Variables**
 
 .. zuul:rolevar:: tox_prefer_python2
-   :default: ``false``
+   :default: False
 
    If tox is not detected, prefer to install tox inside Python 2
    instead of Python 3.
@@ -19,7 +19,7 @@ install directory (i.e., ``pip install --user``).
 **Output Variables**
 
 .. zuul:rolevar:: tox_executable
-   :default: ``tox``
+   :default: tox
 
    After running this role, ``tox_executable`` will be set as the path
    to a valid ``tox``.

roles/tox/README.rst

@@ -24,8 +24,9 @@ Runs tox for a project
 .. zuul:rolevar:: tox_constraints_file
 
    Path to a pip constraints file. Will be provided to tox via
-   TOX_CONSTRAINTS_FILE (deprecated but currently still supported
-   name is UPPER_CONSTRAINTS_FILE) environment variable if it exists.
+   ``TOX_CONSTRAINTS_FILE`` (deprecated but currently still supported
+   name is ``UPPER_CONSTRAINTS_FILE``) environment variable if it
+   exists.
 
 .. zuul:rolevar:: tox_install_siblings
    :default: true