Clarify docstring about weights

[DominikPeters]

The documentation regarding Voter.weight said:

This should not be used as the number of voters with these approved candidates.

I'm not sure what this means, so I've replace it with:

If weight is an integer, the voter is interpreted as if there are weight many voters with the same approval set.

[codecov-commenter]

Codecov Report

Merging #69 (6329cfd) into master (cbd70e7) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master      #69   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
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  Files          11       11           
  Lines        3400     3400           
  Branches     1072     1072           
=======================================
  Hits         3120     3120           
  Misses        161      161           
  Partials      119      119           

Impacted Files	Coverage Δ
abcvoting/preferences.py	100.00% <ø> (ø)

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