Documenting backend upgrades needed for Store in 1.1.0

[mtreinish]

Let just go with documenting this for now as part of the 1.1.x final documentation, I'll convert this into an issue to track. Both as an upgrade note in the final release notes and also as a guidance in the writing a backend section of the docs.

I think for 1.2.x we can look at adding a global flag in the target for something like supports store, etc to provide a high level indicator for this kind of capability above the per instruction look ups. We'd probably need to do this into an opt-in way (so it defaults true) but I think having the capability to express this in the target is what we'll want longer term.

Originally posted by @mtreinish in #12322 (comment)