update RTM rate limiting comment and error message

[benoitlavigne]

Summary

Edit to the comment describing the 3 minutes RTM's reconnect logic
Edit to the error message thrown after 5 rate limited connection attempts to the RTM

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #308 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #308   +/-   ##
=======================================
  Coverage   74.33%   74.33%           
=======================================
  Files          10       10           
  Lines         378      378           
  Branches       67       67           
=======================================
  Hits          281      281           
  Misses         88       88           
  Partials        9        9

Impacted Files	Coverage Δ
slackclient/server.py	85.79% <100%> (ø)	⬆️

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