Fix _ACTC_ChildNum documentation and use it more often in the code

[martinholmer]

The changes in this pull request were suggested by @evtedeschi3 in issue #1665.

Hopefully the new documentation is less confusing and certainly the code is more accurately parameterized.
These changes have no effect on current-law tax results, but the more thorough parameterization of the ACTC function could cause reforms that change _ACTC_ChildNum to produce slightly different (and more accurate) results than they did before these changes.

@MattHJensen @feenberg @Amy-Xu @andersonfrailey @hdoupe @codykallen @evtedeschi3

[codecov-io]

Codecov Report

Merging #1666 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1666   +/-   ##
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  Coverage     100%    100%           
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  Files          37      37           
  Lines        2856    2856           
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  Hits         2856    2856

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[martinholmer]

No concerns or comments, so merging pull request #1666, which responds to bug report in issue #1665.