Improve @ConditionalOnClass javadoc regarding use on @Bean methods

[philwebb]

As suggested in #15578 (comment), we have information in the reference docs that would also be beneficial in the javadoc

[bondolo]

Thank you! This is a great improvement. It will still probably be mysterious for users when they encounter the failure after innocently putting @ConditionalOnClass on a @Bean but the javadoc now adequately explains the problem and a remedy!

[vpavic]

Suggestion for 2.7+: wouldn't it be a good idea to also refer to @AutoConfiguration here, since that's what one would use on something like MyAutoConfiguration these days?

[philwebb]

Good spot @vpavic, I've now fixed that.