[JVMCI] Clarify jdk.vm.ci.code.Architecture.getName javadoc

For historical reasons, the value returned by jdk.vm.ci.code.Architecture.getName is upper case for some architectures (e.g. "AMD64") and lower case for others (e.g. "aarch64", "riscv64"). Ideally they should all be consistent wrt to case but that would be an incompatible change. Instead, the javadoc should be enhanced to point out the inconsistency.