RunOutput(launch:): rename parameter to ‘command’, add documentation.

Removed
```swift
	group.enter()
	defer {
		group.leave()
	}
```

because no matter what happens, "group.leave()" will be called when exiting this scope. As such it serves no purpose. We still need to use ‘group’ for the asynchronous reading of standard error though.

Author: kareman

Sources/SwiftShell/Command.swift

@@ -137,32 +137,27 @@ public final class RunOutput {
 	/// The error from running the command, if any.
 	public let error: CommandError?
 
-	init(launch output: AsyncCommand) {
+	/// Launches command, reads all output from both standard output and standard error simultaneously,
+	/// and waits until the command is finished.
+	init(launch command: AsyncCommand) {
 		var error: CommandError?
 		var stdout = Data()
 		var stderror = Data()
-		let queue = DispatchQueue.global()
 		let group = DispatchGroup()
 
 		do {
 			// launch and read stdout and stderror.
 			// see https://github.com/kareman/SwiftShell/issues/52
-			group.enter()
-			defer {
-				group.leave()
-			}
-			try output.process.launchThrowably()
+			try command.process.launchThrowably()
 
-			if output.stdout.filehandle.fileDescriptor != output.stderror.filehandle.fileDescriptor {
-				group.enter()
-				queue.async {
-					stderror = output.stderror.readData()
-					group.leave()
+			if command.stdout.filehandle.fileDescriptor != command.stderror.filehandle.fileDescriptor {
+				DispatchQueue.global().async(group: group) {
+					stderror = command.stderror.readData()
 				}
 			}
 
-			stdout = output.stdout.readData()
-			try output.finish()
+			stdout = command.stdout.readData()
+			try command.finish()
 		} catch let commandError as CommandError {
 			error = commandError
 		} catch let error {
@@ -173,7 +168,7 @@ public final class RunOutput {
 
 		self.rawStdout = stdout
 		self.rawStderror = stderror
-		self.output = output
+		self.output = command
 		self.error = error
 	}
 
@@ -341,7 +336,7 @@ public class PrintedAsyncCommand {
 	}
 }
 
-/** Output from the 'runAsync' methods. */
+/** Output from the 'runAsync' commands. */
 public final class AsyncCommand: PrintedAsyncCommand {
 	public let stdout: ReadableStream
 	public let stderror: ReadableStream