Resolve conflicts, add doc comments

Author: MaxDesiatov

Sources/PackageModel/Destination.swift

@@ -243,7 +243,7 @@ extension Destination {
                 binDir: destination.binDir,
                 extraCCFlags: destination.extraCCFlags,
                 extraSwiftCFlags: destination.extraSwiftCFlags,
-                extraCPPFlags: destination.extraCPPFlags
+                extraCXXFlags: destination.extraCPPFlags
             )
         } else if version.version == 2 {
             let destination = try decoder.decode(path: path, fileSystem: fileSystem, as: DestinationInfoV2.self)
@@ -255,7 +255,8 @@ extension Destination {
                 binDir: destination.binDir.absolutePath(relativeTo: destinationDirectory),
                 extraCCFlags: destination.extraCCFlags,
                 extraSwiftCFlags: destination.extraSwiftCFlags,
-                extraCPPFlags: destination.extraCPPFlags
+                // maintaining `destination.extraCPPFlags` naming inconsistency for compatibility.
+                extraCXXFlags: destination.extraCPPFlags
             )
         } else {
             throw DestinationError.invalidSchemaVersion
@@ -264,6 +265,10 @@ extension Destination {
 }
 
 fileprivate extension FilePath {
+    /// Converts `FilePath` to `AbsolutePath`, in relation to other `AbsolutePath` if possible.
+    /// - Parameter directory: an absolute path to a directory used to calculate a resulting `AbsolutePath`.
+    /// - Returns: trivially converts `self` to `AbsolutePath` as a string if `self` is an absolute path.
+    /// Otherwise, if `self` is a relative path, returns `self` appended to the `directory` argument.
     func absolutePath(relativeTo directory: AbsolutePath) throws -> AbsolutePath {
         if isRelative {
             return directory.appending(RelativePath(string))