* Clarify Cranelift's design with respect to mid-level optimization.
Cranelift doesn't currently do much mid-level optimization, however it
is something we're thinking about, so remove text describing it as out of
scope, and add more text explaining the vision for how it would fit into
the overall system.
While the specifics of these terms are debatable, "IR" generally
isn't incorrect in this context, and is the more widely recognized
term at this time.
See also the discussion in #267.
Fixes#267.
* Clarify that extended basic blocks are abbreviated as EBB.
* Fix typo.
* Fix a typo.
* Fix typos.
* Use the same phrase to indicate scalar-only as other places in the doc.
* Mention that `band_imm` and friends are scalar-only.
And mention that they're equivalent to their respective
non-immediate-form counterparts.
