The check_extra_config function from linux-info.eclass currently has three different severety levels:
CONFIG_CHECK="CFG" with ERROR_CFG="foo" is a hard error, will die.
CONFIG_CHECK="~CFG" with ERROR_CFG="foo" is a "soft error", calls eerror.
CONFIG_CHECK="~CFG" with WARNING_CFG="foo" is a warning, calls ewarn.
The distinction between the latter two isn't anywhere in the documentation. There is a piece of documentation about ERROR_<CFG> variables, but none at all about WARNING_<CFG> variables.
This can lead to confusion, bug #407547 comment #3 being an example. So please fix the documentation, so it properly documents all possible behaviour.