doc: update Gentoo package availability info

Signed-off-by: Andrey Utkin <andrey_utkin@gentoo.org>
7 jobs for master in 2 minutes and 38 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #22504
allowed to fail
pylint-plugins

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pytest-3.5

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pytest-3.6

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passed #22503
pytest-latest

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passed #22500
security-check

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  Build
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www
build-doc

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build-ubuntu

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