update subset docs - no support for migration table

[mufernando]

I forgot to update the documentation after I removed the support for subsetting the Migration Table in #663. I followed the same that was done for TableCollection.simplify() -- that is added a note (it was there already) in the C API and removed any mentions of migration in the Python API.

[codecov]

Codecov Report

Merging #690 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #690   +/-   ##
=======================================
  Coverage   87.71%   87.71%           
=======================================
  Files          23       23           
  Lines       17810    17810           
  Branches     3526     3526           
=======================================
  Hits        15622    15622           
  Misses       1073     1073           
  Partials     1115     1115           

Flag	Coverage Δ
#c_tests	88.95% <ø> (ø)
#python_c_tests	91.26% <ø> (ø)
#python_tests	98.99% <ø> (ø)

Impacted Files	Coverage Δ
python/tskit/trees.py	98.21% <ø> (ø)

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[petrelharp]

Lgtm!