add a comment referring to the SR-14665

Reference to [SR-14665](apple/swift#57016) links to more explanation on why the manual `==` implemenatation is necessary.

Sources/SymbolKit/SymbolGraph/SemanticVersion/SemanticVersion+Comparable.swift

@@ -10,6 +10,7 @@
 
 extension SymbolGraph.SemanticVersion: Comparable {
     // Although `Comparable` inherits from `Equatable`, it does not provide a new default implementation of `==`, but instead uses `Equatable`'s default synthesised implementation. The compiler-synthesised `==`` is composed of [member-wise comparisons](https://github.com/apple/swift-evolution/blob/main/proposals/0185-synthesize-equatable-hashable.md#implementation-details), which leads to a false `false` when 2 semantic versions differ by only their build metadata identifiers, contradicting SemVer 2.0.0's [comparison rules](https://semver.org/#spec-item-10).
+    // [SR-14665](https://github.com/apple/swift/issues/57016)
     /// Returns a Boolean value indicating whether two semantic versions are equal.
     /// - Parameters:
     ///   - lhs: A semantic version to compare.