Add tox job descriptions

tox.ini

@@ -9,6 +9,8 @@ commands =
     pytest {posargs}
 
 [testenv:coverage]
+description =
+    Measure test coverage and generate HTML report. Pass `xml` or `lcov` to specify different format.
 deps =
     {[testenv]deps}
     coverage==7.*
@@ -18,6 +20,8 @@ commands =
     coverage {posargs:html}
 
 [testenv:code]
+description =
+    Check code with flake8 and bandit.
 skip_install = true
 deps =
     flake8==6.*
@@ -30,6 +34,8 @@ commands =
 max-line-length = 88
 
 [testenv:changelog]
+description =
+    Collect changelog fragments. Pass `create` to create a new fragment.
 skip_install = true
 deps =
     scriv==1.*