clarification to keep_intervals docs

[petrelharp]

On the msprime mailing list, @mnavascues rightly asks:

I am using SLiM and pyslim+msprime (for recapitation) for simulating
whole human genomes. I want to apply msprime independently to smaller
chuncks of genome (e.g. chromosome arms) and wanted to use
keep_interval() as suggested by Peter Ralph
(https://groups.google.com/forum/#!topic/slim-discuss/UIwmOPuv9Ww).

However, I am not sure I am doing it right, the resulting tree sequence
seems to be of the same size of the original sequence:

 >>> import msprime
 >>> tree_sequence = msprime.simulate(sample_size=6, length=50e3, Ne=1000)
 >>> tree_sequence.sequence_length
50000.0
 >>> tree_sequence.keep_intervals(np.array([[0,10]])).sequence_length
50000.0

Shouldn't this give me a sequence length of 10?

But, no: this is something that we discussed. Hopefully, this change should clear up this confusion?

[petrelharp]

How's this look, @mnavascues?

[codecov]

Codecov Report

Merging #889 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #889   +/-   ##
=======================================
  Coverage   93.52%   93.52%           
=======================================
  Files          25       25           
  Lines       19932    19932           
  Branches      789      789           
=======================================
  Hits        18641    18641           
  Misses       1259     1259           
  Partials       32       32           

Impacted Files	Coverage Δ
python/tskit/trees.py	98.13% <ø> (ø)

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[mnavascues]

How's this look, @mnavascues?

Looks good to me. Thanks!