Update custom_authentication_provider.rst

improved description of the getKey method
minor #3821 Update custom_authentication_provider.rst (leberknecht)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3821).

Discussion
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Update custom_authentication_provider.rst

improved description of the getKey method

Commits
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89c2d88 Update custom_authentication_provider.rst