docs: improve exception matching

[jesusarell]

Explicitly state that the string argument its checked to be contained in the error message instead of to be the error message.

Summary

Test plan

[SimenB]

thanks! Can you update the other versions of this doc as well?

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[jesusarell]

Done! Waiting for CLA check

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[SimenB]

thanks!

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