Document incompatibility of VerifyNone with t.Parallel

[JacobOaks]

It's a known issue that goleak is incompatible with tests being run in parallel.

• https://github.com/uber-go/goleak/issues/16
• https://github.com/uber-go/goleak/issues/83

Unfortunately, there is no real solution to this issue.

• https://github.com/uber-go/goleak/issues/16#issuecomment-575020250
• https://github.com/uber-go/goleak/issues/83#issuecomment-1487300420
• https://github.com/uber-go/goleak/issues/83#issuecomment-1755316283

This PR at least documents the incompatibility and suggests using VerifyTestMain instead.

[codecov[bot]]

Codecov Report

Merging #107 (7a1ce0a) into master (3e8744f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #107   +/-   ##
=======================================
  Coverage   96.58%   96.58%           
=======================================
  Files           5        5           
  Lines         234      234           
=======================================
  Hits          226      226           
  Misses          5        5           
  Partials        3        3           

Files	Coverage Δ
leaks.go	100.00% <ø> (ø)

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