Updated jsdocs to be more descriptive

[WesleyClements]

The current jsdocs aren't descriptive enough for typescript to understand that this middleware is compatible with Koa.
I've updated them to specify that compose takes and returns a Koa.Middleware.

[codecov]

Codecov Report

Merging #150 (fac13ad) into master (37d083f) will not change coverage.
The diff coverage is 100.00%.

❗ Current head fac13ad differs from pull request most recent head 6984428. Consider uploading reports for the commit 6984428 to get more accurate results

@@            Coverage Diff            @@
##            master      #150   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           17        17           
  Branches         5         5           
=========================================
  Hits            17        17           

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

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