docs: document that --tarball does not support multi-platform images

## Summary

`ko build --tarball` does not currently support multi-platform image indexes, but the reference docs only describe the flag as:

> File to save images tarballs

This makes it look like `--tarball` should work with the same multi-platform builds that `ko` otherwise supports.

## Current Behavior

A multi-platform build with `--tarball` fails because the native tarball publisher only supports `v1.Image`, not `v1.ImageIndex`.

Related source:

- `pkg/publish/tarball.go` downcasts the build result to `v1.Image`, with a source comment stating there is no way to write an index to a tarball:
- the tarball is written on `Close` via `tarball.MultiRefWriteToFile`:

Related prior issues:

-
-

## Suggested Docs Change

Document the limitation wherever `--tarball` is described, for example:

> `--tarball` writes Docker-save-style image tarballs for single-platform images. Multi-platform image indexes are not supported by the tarball publisher; use `--oci-layout-path` for multi-platform output.

This would help users understand when to use `--tarball` versus `--oci-layout-path`.

Start with the reference documentation entries that describe --tarball, then read pkg/publish/tarball.go and the related issues for context. Update every relevant description to state that multi-platform image indexes are unsupported and point users to --oci-layout-path; done means the limitation and alternative are clear wherever the flag is documented.

Written by the indexing model from the issue text.