[skip changelog] Make the command documentation of cache clean more clear (arduino#1135)

The terms "cache" and "clean" might bring compilation to mind, but the `arduino-cli cache clean` command has nothing to
do with compilation. Many users will likely not even be aware of the existence or purpose of the `directories.downloads`
folder. For this reason, it's important to make the purpose of this command clear.

Author: per1234

cli/cache/clean.go

@@ -28,8 +28,8 @@ import (
 func initCleanCommand() *cobra.Command {
 	cleanCommand := &cobra.Command{
 		Use:     "clean",
-		Short:   "Clean arduino cache.",
-		Long:    "Clean the files i.e. `~/arduino15/staging` in Linux.",
+		Short:   "Delete Boards/Library Manager download cache.",
+		Long:    "Delete contents of the `directories.downloads` folder, where archive files are staged during installation of libraries and boards platforms.",
 		Example: "  " + os.Args[0] + " cache clean",
 		Args:    cobra.NoArgs,
 		Run:     runCleanCommand,