Update Semigroup docs for Map

[codingismy11to7]

I didn't understand that the values in Maps were also combined when two maps are combined - I thought the "hello" -> 2 entry was simply replacing the "hello" -> 0 entry in the first map. Giving the first map's "hello" a value of 1 results in the combined map having a "hello" -> 3 entry, showing that the values were combined.

After understanding that, I realized why my code couldn't find an implicit Semigroup for my Map - I didn't have an implicit Semigroup for my map's values.

Thank you for contributing to Cats!

This is a kind reminder to run sbt prePR and commit the changed files, if any, before submitting.

[LukaJCB]

Thanks, this does make it a bit easier to see what's going on :)

[codecov-io]

Codecov Report

Merging #3071 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3071   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files         368      368           
  Lines        6979     6979           
  Branches      184      184           
=======================================
  Hits         6523     6523           
  Misses        456      456

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c138a50...d6064c3. Read the comment docs.