Update CLI documentation for --path.install (elastic#3863)

`--path.install` has no effect since v8.6.0, removing it is a breaking
change because it would prevent the Elastic-Agent from running if
`--path.install` is passed.

Instead of removing it, this commit updates its documentation to
inform it is deprecated and has no effect.

internal/pkg/agent/application/paths/common.go

@@ -59,7 +59,7 @@ func init() {
 	fs.StringVar(&configPath, "path.config", configPath, "Config path is the directory Agent looks for its config file")
 	fs.StringVar(&configFilePath, "c", DefaultConfigName, "Configuration file, relative to path.config")
 	fs.StringVar(&logsPath, "path.logs", logsPath, "Logs path contains Agent log output")
-	fs.StringVar(&installPath, "path.install", installPath, "Install path contains binaries Agent extracts")
+	fs.StringVar(&installPath, "path.install", installPath, "DEPRECATED, setting this flag has no effect since v8.6.0")
 
 	// enable user to download update artifacts to alternative place
 	// TODO: remove path.downloads support on next major (this can be configured using `agent.download.targetDirectory`)