docs on missing data and coordinate systems, closes #1584

[petrelharp]

In perusing #1584 I realized we hadn't written down this thing about the coordinate system being fixed even when we simulate a chunk. Now it's written down - hopefully without assuming the reader understands tskit too much?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (16497b3) to head (27221c7).
Report is 5 commits behind head on main.

Additional details and impacted files

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- Coverage   99.84%   99.84%   -0.01%     
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  Files         130      130              
  Lines        4410     4405       -5     
  Branches      607      606       -1     
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- Hits         4403     4398       -5     
  Misses          3        3              
  Partials        4        4              

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[petrelharp]

Here's the result - maybe too verbose?

[gregorgorjanc]

Looks good to me. I don’t find it too verbose - if docs are very terse then the reader already needs to know lots about tskit.

[petrelharp]

Thanks! Well, I think these docs are aiming to get the reader to - say - VCFs as quickly and painlessly as possible. Hopefully people doing that will realize they don't need to worry about that. Hm: maybe I'll just add an explicit note saying so.

[petrelharp]

Added