More docs on `disk_backend` in disk create

[david-crespo]

This shows up in the docs site and in LSP hovers in the SDKs. We need to explain a bit about the difference and strongly encourage users to pick distributed unless they are sure they want local.

omicron/nexus/types/src/external_api/params.rs

Lines 2224 to 2225 in e6961bc

	/// The source for this `Disk`'s blocks
	pub disk_backend: DiskBackend,