Document that ping_interval should be >= wait_timeout

[anarthal]

Fix defect in connection_pool docs

close #290

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (eba723f) to head (f987fbf).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #373   +/-   ##
========================================
  Coverage    99.04%   99.04%           
========================================
  Files          143      143           
  Lines         7157     7157           
========================================
  Hits          7089     7089           
  Misses          68       68           

Files with missing lines	Coverage Δ
include/boost/mysql/connection_pool.hpp	100.00% <ø> (ø)
include/boost/mysql/pool_params.hpp	100.00% <ø> (ø)

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