Improve documentation related to overriding validation groups

[darrylhein]

I've noticed that the page regarding overriding validation groups is quite short and doesn't really provide the full details: https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/doc/overriding_validation.rst

The piece of the details that I feel is missing is how to actually change which validation groups you want applied to a form, specifically after creating new groups.

Before I send a pull request, I wanted to check if this was something that people feel would be valuable. I personally eventually figured it out by looking in the configuration in dependency injection...not the easiest thing to figure out.

I'm good to make an attempt at adding to the page.

[XWB]

Always 👍 for improving documentation.

[XWB]

Any updates @darrylhein ?