cylc_lang: nested meta sections

[oliver-sanders]

Low priority bug (more a limitation than a bug) easier just to document for now in the (hopefully) unlikely case that we will need this (famous last words I will likely eat if we start giving Cylc plugins configuration sections...).

As of #50 only settings explicitly defined in meta sections are documented.

Example:

[platforms]
    [[<platform name>]]
        host
       [[[selection]]]
           method

    [[localhost]]
        host = localhost

The above example will document fine, the overwritten host setting will appear correctly.

However, if we wanted to overwrite the [selection]method for localhost that would not appear in the docs.

I think fixing this would require making cylc.flow.parsec.config.ConfigNode.walk aware of the meta keyword.

See #50