Editorials about AS and RS on the same entity or on different ones

[peppelinux]

          > Obviously very wrong how it is, so approving.

Probably we should mention that the Credential Issuer may optionally also be an AS.

Can we consolidate the information scattered throughout the text into a single section or paragraph, in another PR? Additionally, we may eliminate repetitions and distinctly differentiate between AS != RS and AS+RS.

• https://github.com/openid/OpenID4VCI/blob/main/openid-4-verifiable-credential-issuance-1_0.md#oauth-20
• https://github.com/openid/OpenID4VCI/blob/main/openid-4-verifiable-credential-issuance-1_0.md?plain=1#L223C1-L223C587
• https://github.com/openid/OpenID4VCI/blob/main/openid-4-verifiable-credential-issuance-1_0.md?plain=1#L106C1-L106C332
• https://github.com/openid/OpenID4VCI/blob/main/openid-4-verifiable-credential-issuance-1_0.md?plain=1#L222C1-L223C381

Originally posted by @peppelinux in https://github.com/openid/OpenID4VCI/issues/299#issuecomment-2084602151