Currently there is no mechanism to identify totally undocumented modules
in the coverage builder, unlike with partially documented modules.
Resolve this by introducing a new ``coverage_modules`` config option.
This is a list of modules that should be documented somewhere
within the documentation tree.
Any modules that are specified in the configuration value but
are not documented anywhere will result in a warning.
Likewise, any modules that are not in the config option but
are documented somewhere will result in a warning.
Signed-off-by: Stephen Finucane <stephen@that.guru>
