`tskit.write_ms` is not documented

[petrelharp]

The top-level tskit.write_ms() method, added in #854, does not appear in the online documentation anywhere, although it is documented (eg help(tskit.write_ms) works).

I think this should go in a subsection of python-api; perhaps a separate top-level section labeled "Output to other formats" (that would also mention TreeSequence.write_vcf) for maximum discoverability.

[benjeffery]

This was intentional - there are aspects of the format that we were not sure of and wanted more feedback that we were doing the right thing. It was also debated if methods such as this actually belong in tsconvert.

[petrelharp]

Ah - missed that, sorry. I think this would make sense in tsconvert. I'm going to leave this open, though, so we remember to do it.

[andrewkern]

hey if y'all want feedback about the ms output format-- i might be able to help

[jeromekelleher]

Not sure it's worth the effort? Do people want/need this today?