Fix list of macro roles that need "names:" argument

[daltonclaybrook]

This PR fixes a documentation bug stating that the names: argument is required for @attached macros with the accessor role. In reality, this argument is not required for accessor macros.

Fixes: #374

[amartini51]

@daltonclaybrook Were you able to confirm that this is correct and expected behavior from Swift Syntax, as suggested on the GitHub issue? If not, do you know who to talk to, or can I help you find a technical reviewer? Thanks!

Marking this as a draft, pending tech review.

[hborla]

@daltonclaybrook is correct; attached accessor macros do not require a names argument. The names argument is only required if the accessor macro generates property observers. This behavior is covered by SE-0389 in the section on accessor macros here: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0389-attached-macros.md#accessor-macros