docs: using: full custom handler is in manifest.raucm not in system.conf
docs: using: full custom handler is in manifest.raucm not in system.conf

We should not confuse the reader more than necessary where we already
have two things we call 'handler'.

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc