Pool docs

[Ethan-Arrowood]

I know this page might seem redundant, but I think it is the best way to document Pool. This clearly shows the user what methods, properties, and events are available to them, and provides easy links to the relevant part of the Client documentation.

Reviewers:

If there is anything specific we should highlight for any of the properties or methods in regards to descriptions we should include that. For example, we may want to include descriptions for properties like pending, busy, etc. specifying how they are evaluated for the pool of clients.

[codecov-io]

Codecov Report

Merging #565 (7b1542b) into master (3d9cb69) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #565   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files          16       16           
  Lines        1398     1400    +2     
=======================================
+ Hits         1392     1394    +2     
  Misses          6        6           

Impacted Files	Coverage Δ
lib/node/http-parser.js	100.00% <0.00%> (ø)

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[mcollina]

lgtm