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MINOR: [R] Update R packaging steps to reflect current release process (#40651)

### Rationale for this change

R release process has changed a little and needs documenting

### What changes are included in this PR?

Documented changes

### Are these changes tested?

No

### Are there any user-facing changes?

No

Lead-authored-by: [Nic Crane <thisisnic@gmail.com>
Co-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>