I agree with what you are saying in that it needs to be called out that time needs to be specifically set aside in agile / sprint planning for mentoring. My manager understood this and I was allocated at least a day sprint to just 'mentor' my junior dev through a 8 months rotational stint in the team. Mentorship is very rewarding and I found out that I got just as much out of the relationship as they did. It also helped that for the first 4 weeks I was allowed to run her through a self lead learning program that I designed for them. Giving a new hire out of univiristy time to learn enough of what they needed to tackle some high value work just made the final outcome so much more successful.