Extend doc about subexpression calls

[jaynetics]

I expected /(.)\g<1>\1/ to match ABA. Perhaps the docs should state in some way that the matched text used for backreferences is determined by the closest occurrence of the target group - be it a subexp call or the original group.

[codecov-io]

Codecov Report

Merging #105 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
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  Coverage   82.86%   82.86%           
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  Files          62       62           
  Lines       12827    12827           
=======================================
  Hits        10629    10629           
  Misses       2198     2198

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