Cranelift: extend docs on Inst to discuss `call` instructions

[iximeow]

the docs on Inst note that the type is returned by non-resultful instructions built from InstBuilder, but did not note that it is also returned by call and call_indirect. if you're trying to learn and use Cranelift by following the docs, this means you'd follow a doc link to Inst that implies that call does not return a value - this is actively misleading, since you'd want to use the returned Inst to find exactly those returned values!

so, this adds a few sentences talking about the case of call Insts.

[iximeow]

i forget the etiquette here since i've pushed a change since your review, but i still have a shiny green merge button. CI's gotta churn anyway, so i'll let you have the last word on if this is good for a squash merge still - i assume so :D