Document BearerTokenResolver in reference

[jgrandja]

There is no documentation for BearerTokenResolver and it's default implementation DefaultBearerTokenResolver in the reference. We should document and provide code samples on how to use and customize.

[kostya05983]

Hello @jgrandja , I can take this one, If I understand correctly, I should add docs in docs module of spring security, shouldn't I? And where can I put code samples of this?