Improve measurement details in README for phpunit 10+

[mvorisek]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.86%. Comparing base (f7eadb6) to head (855834f).
Report is 261 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #452   +/-   ##
=========================================
  Coverage     69.86%   69.86%           
  Complexity      110      110           
=========================================
  Files            26       26           
  Lines           521      521           
=========================================
  Hits            364      364           
  Misses          157      157           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mvorisek]

Firstly, thank you for merging the #380 PR!

I did my best to improve the README for new users, especially to stress:

• the before/after class test methods are measured only when phpunit process isolation is enabled
• remove the notes telling the durations might vary on a tests order, this is not true

[mvorisek]

@localheinz is this PR welcomed?