with_diagnostics now passes through unknown signatures

[anarthal]

This makes asio::as_tuple and asio::redirect_error usable as partial completion tokens with any_connection and connection_pool.
Removed leftover comment in the pipeline example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (272533c) to head (a044e96).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #377   +/-   ##
========================================
  Coverage    99.04%   99.05%           
========================================
  Files          143      143           
  Lines         7157     7161    +4     
========================================
+ Hits          7089     7093    +4     
  Misses          68       68           

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

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