Clarify the nature of AU code systems that extend Core code systems

[RichardTON]

The specification of IdentifierTypeAU can be misread as a local redefinition of V2 table 0203 (Identifier Type).
See Zulip chat

Make the description clearer about the relationship between it and the actual v2-0203 table.

This applies to the following HL7 AU code HL7 AU systems

• ActCodeAU
• IdentifierTypeAU
• DegreeLicenseCertificateAU
• ProviderRoleAU
• LocationTypePhysicalAU

[dt-r]

IdentifierTypeAU fixed in commit:
fabb266