[Docs] Clearly state that timeout value applies to a test, not test session

[pytest-timeout-bot]

Original report by Андрей Парамонов (Bitbucket: aparamon, GitHub: aparamon).

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Currently, docs say:

This is a plugin which will terminate tests after a certain timeout. When doing so it will show a stack dump of all threads running at the time. This is useful when running tests under a continuous integration server or simply if you don’t know why the test suite hangs.

It seems that the plugin will terminate the long-running test suite (i.e. timeout applies to the test session), however in reality timeout applies to every test item, independently.

It is proposed to re-phrase:

This is a plugin which terminates a test after a certain timeout.

[pytest-timeout-bot]

Original comment by Floris Bruynooghe (Bitbucket: flub, GitHub: flub).

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Seems good, would you like to do this in a PR?