Document how to update toolchains for existing easyconfigs

[Flamefire]

This question comes up every now and then so I described the process I usually use.

[ocaisa]

The one thing missing is a mention of checksums and how to update them, perhaps it is as simple as --inject-checksums --force with a link to the more detailed docs on that topic?

[ocaisa]

Thanks a lot for this @Flamefire, I think this is a really valuable addition.