Add doc alias "single" to "exactly_one"

[nicopap]

"Single" seems like an obvious name for "exactly_one", so I feel like adding this alias makes it more discoverable.

It's notable that "single" doesn't appear in the documentation for exactly_one either.

I found a few issues that show that indeed this change might help people:

• only / single #595
• single? #334
• Feature request: single.  #566

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (6814180) to head (51a708c).
⚠️ Report is 212 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1125      +/-   ##
==========================================
- Coverage   94.38%   93.90%   -0.49%     
==========================================
  Files          48       52       +4     
  Lines        6665     6680      +15     
==========================================
- Hits         6291     6273      -18     
- Misses        374      407      +33     

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[phimuemue]

Very good, indeed. Thanks.