clarify add_dfe docs (closes #1580)

[petrelharp]

Also fixed the code display:

tldr; is that stdpopsim, like msprime/tskit, uses [a,b); while SLiM uses [a,b].

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (0ab99e0) to head (ce075b0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1663   +/-   ##
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  Coverage   99.84%   99.84%           
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  Files         136      136           
  Lines        4653     4653           
  Branches      468      468           
=======================================
  Hits         4646     4646           
  Misses          3        3           
  Partials        4        4           

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[mufernando]

Important clarification! LGTM.