Severity level error is still listed in the documentation for the checks that were recently demoted to warnings

In https://github.com/google/error-prone/commit/230ad26ec1d308f6387e7549612d2b4baf244cf2
the two checks were demoted to severity level warning, but it seems that the documentation was missed to be adapted: [1], [2].

[1] https://errorprone.info/bugpattern/ExtendsAutoValue
[2] https://errorprone.info/bugpattern/RefersToDaggerCodegen