Nominally reference the IRI specification in documentation

[nvmkuruc]

Description of Change(s)

The URI specification nominally referenced throughout the Ar documentation restricts the character set to ASCII characters. This update to documentation clarifies that--

• Resolvers registered via uriSchemes may also accept UTF-8 encoded IRIs (a unicode supporting resource identifier specification)
• Resolvers working with protocols that only accept URIs may (but are not required to) support IRIs encoded in layers and handle percent encoding idempotently at the resolver level
• USD does not validate URI or IRI paths beyond what's needed to dispatch via a resource identifier's scheme component. Resolvers handle their own parsing, validation, and normalization of asset paths.

This change also converts the invalid my_scheme URI/IRI scheme to my-scheme.

Fixes Issue(s)

• Support for Unicode UTF-8 Identifiers #2120

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-8385