Increase discoverability of `merge_join_by` (Unix-like `comm`)

[Philippe-Cholet]

Fixes #963

I suggest we follow the same policy as the stdlib: https://std-dev-guide.rust-lang.org/policy/doc-alias.html And I agree with @VorpalBlade that comm is a fitting alias.

PS: Doc aliases requires rust 1.48 so we are good now.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.64%. Comparing base (6814180) to head (84ce7cc). Report is 111 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
+ Coverage   94.38%   94.64%   +0.25%     
==========================================
  Files          48       49       +1     
  Lines        6665     7229     +564     
==========================================
+ Hits         6291     6842     +551     
- Misses        374      387      +13     

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