tox: Add documentation for all targets

This should make 'tox -av' more helpful.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -1,9 +1,15 @@
 [tox]
-minversion=2.0
-envlist=docs,flake8,mypy,coverage,py{27,34,35,36,py},du{11,12,13,14}
+minversion = 2.0
+envlist = docs,flake8,mypy,coverage,py{27,34,35,36,py},du{11,12,13,14}
 
 [testenv]
-passenv = https_proxy http_proxy no_proxy
+passenv =
+    https_proxy http_proxy no_proxy
+description =
+    py{27,34,35,36,py}: Run unit tests against {envname}.
+    du{11,12,13,14}: Run unit tests with the given version of docutils.
+    coverage: Run code coverage checks.
+
 # TODO(stephenfin) Replace this with the 'extras' config option when tox 2.4 is
 # widely available, likely some time after the Ubuntu 18.04 release
 #
@@ -21,22 +27,30 @@ commands=
     {envpython} -Wall tests/run.py --durations 25 {posargs}
 
 [testenv:flake8]
-commands=flake8
+description =
+    Run style checks.
+commands =
+    flake8
 
 [testenv:pylint]
-deps=
+description =
+    Run source code analyzer.
+deps =
     pylint
     {[testenv]deps}
-commands=
+commands =
     pylint --rcfile utils/pylintrc sphinx
 
 [testenv:mypy]
-basepython=python3
-deps=
+description =
+    Run type checks.
+deps =
     mypy
 commands=
     mypy sphinx/
 
 [testenv:docs]
-commands=
+description =
+    Build documentation.
+commands =
     python setup.py build_sphinx {posargs}