Systematic documentation for YAML object attributes

[smheidrich]

Thanks for this fork, I couldn't be bothered to navigate SourceForge's horrible UI in 2020 just to report this:

The documentation for the various attributes that can be set on YAML objects to influence the library's behavior is quite sparse/nonexistent. Some of these options were just carried over from PyYAML, so perhaps for those it's not necessary to have extra documentation here, but e.g. things like sort_base_mapping_type_on_output are very useful, undocumented in ruamel.yaml, and have no PyYAML equivalent AFAICT. So for this and the other attributes that fulfill these criteria, we should add some docs.

[jwilson8767]

Thanks for creating this issue! I, too, have been annoyed by the lack of documentation and hope we can improve exactly this sort of thing! We're still working on migrating the existing docs to a more useful form, but in the meantime if you wanted to drop literally any comments onto some of those attributes and submit a PR, it would be really helpful!

Heck, if you even take a few minutes to type up the options you have found helpful in the past I'll make a point to make sure they end up in docs in the future.

[lyz-code]

I don't know where to put it in the docs, so I'm adding here a table and I'll go on completing it while I use the library:

YAML() attributes:

Property	Type	Description
explicit_start	boolean	Add --- at the start of the source
sequence_indent	int