Update the skip message with an action for creating your own documentation for a target

Author: cmcgee1024

Sources/Commands/PackageCommands/GenerateDocumentation.swift

@@ -270,7 +270,7 @@ struct GenerateDocumentation: AsyncSwiftCommand {
             }
 
             guard doccCatalogDir != nil || symbolGraphDir != nil else {
-              print("Skipping \(product.name) because there is no DocC catalog and there is no symbol graph that could be generated for it.")
+              print("Skipping \(product.name) because there is no DocC catalog and there is no symbol graph that could be generated for it. You can add your own documentation for this executable product by adding a documentation directory with the '.docc' file extension and your own DocC formatted markdown files in the module for this product.")
               continue
             }
 
@@ -310,7 +310,7 @@ struct GenerateDocumentation: AsyncSwiftCommand {
             }
 
             guard doccCatalogDir != nil || swiftCommandState.fileSystem.exists(symbolGraphPath) else {
-              print("Skipping \(module.name) because there is no DocC catalog and there is no symbol graph that could be generated for it.")
+              print("Skipping \(module.name) because there is no DocC catalog and there is no symbol graph that could be generated for it. You can write your own documentation for this target by creating a directory with a '.docc' file extension and adding DocC formatted markdown files.")
               continue
             }