Add more information about whitelisting.

[Mighty683]

Is your feature request related to a problem? Please describe.

pseudo-elements like :before are considered as children e.g selector:
i.flag.us:before

When I wanted whitelist selectors connected with flag icons which are build like one above I
used whitelistPatterns: [/flag/] pattern but it didn't work.

i.flag - selector passed test
i.flag.us:before - selector was purged

I had to use: whitelistPatternsChildren: [/flag/]

Describe the solution you'd like
It would be good if such information would be added in documentation, maybe add an example?

[Ffloriel]

yes, it makes sense to add additional information or an example as it is not obvious