Clarify `llvm_unreachable` after exhaustive switch is needed to silence MSVC C4715, so we don't accidentally remove it. Standardize some assertion messages.
Rename "emitters" to "cloners", for consistency: - `JVPEmitter` -> `JVPCloner` - `VJPEmitter` -> `VJPCloner` - `PullbackEmitter` -> `PullbackCloner` Improve `PullbackCloner` documentation. - Document previously undocumented methods. - Update outdated documentation. - For adjoint value accumulation helpers: rename "buffer access" occurrences to "address". Pullback generation no logner uses buffer accesses (`begin_apply`).
