Update documentation on multiple calls of metafunc.parametrize

[ghost]

Fix #1548

I wanted to add some code to demonstrate but afraid it would be an overkill and make the doc too verbose.

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[coveralls]

Coverage remained the same at 91.805% when pulling e4ccdac on xuanluong:issue-1548-docs-multiple-calls-metafunc-parametrize into bee2c86 on pytest-dev:master.

[coveralls]

Coverage remained the same at 91.805% when pulling 696c702 on xuanluong:issue-1548-docs-multiple-calls-metafunc-parametrize into bee2c86 on pytest-dev:master.

[nicoddemus]

Thanks!