Clarify doc comment for OutOfProcessReferenceResolver.declarationFragments

These declaration fragments are directly used to populate `TopicRenderReference.fragmentsVariants` [1], which is described as "The abbreviated declaration of the symbol to display in links" [2].

This change clarifies that the declaration fragments are expected to be the abbreviated declaration fragments.

[1]: https://github.com/swiftlang/swift-docc/blob/65aaf926ec079ddbd40f29540d4180a70af99e5e/Sources/SwiftDocC/Infrastructure/External%20Data/OutOfProcessReferenceResolver.swift#L169
[2]: https://github.com/swiftlang/swift-docc/blob/65aaf926ec079ddbd40f29540d4180a70af99e5e/Sources/SwiftDocC/Model/Rendering/References/TopicRenderReference.swift#L50-L53

Author: anferbui

Sources/SwiftDocC/Infrastructure/External Data/OutOfProcessReferenceResolver.swift

@@ -576,6 +576,8 @@ extension OutOfProcessReferenceResolver {
         /// Information about the platforms and their versions where the resolved node is available, if any.
         public let platforms: [PlatformAvailability]?
         /// Information about the resolved declaration fragments, if any.
+        ///
+        /// This is expected to be an abbreviated declaration for the symbol.
         public let declarationFragments: DeclarationFragments?
 
         // We use the real types here because they're Codable and don't have public member-wise initializers.