Clarify `backend=backend` runs things in job mode for v2 primitives

[javabster]

If you set backend=backend in a v2 primitive it will run the program in job mode, even if it's inside a batch/session context

with Batch(backend=backend):
    sampler = SamplerV2(backend=backend)

so in general we should emphasise to users that they should not do this and instead use this formats:

with Batch(backend=backend):
    sampler = SamplerV2()

(note session=batch is confusing and will likely be changed in future)

[arujjval]

@javabster I want to take this one

[jyu00]

(note session=batch is confusing and will likely be changed in future)

Yeah we are deprecating both session and backend parameters in primitive __init__ to avoid this confusing in the next release (hopefully coming out this week). The replacement is mode.

[javabster]

thanks @arujjval go ahead and work on this, make sure to check out the README for contribution instructions before you get started.

@jyu00 for now we'll just mention the version without session or backend and then update with mode in a later PR