Fix some errors and docs

index.js

@@ -100,7 +100,7 @@ export class Recorder {
 
 	throwIfNotStarted() {
 		if (this.recorder === undefined) {
-			throw new Error('Call `.startRecording()` first');
+			throw new Error('Recording not started yet');
 		}
 	}
 

readme.md

@@ -208,6 +208,23 @@ A computer with Intel 6th generation processor or newer is strongly recommended
 
 The [`proRes422` and `proRes4444`](https://documentation.apple.com/en/finalcutpro/professionalformatsandworkflows/index.html#chapter=10%26section=2%26tasks=true) codecs are uncompressed data. They will create huge files.
 
+##### extension
+
+Type: `string`\
+Default:
+
+- `'m4a'` for [audio](#recorderstartrecordingaudiooptions) recordings
+- `'mov'` for `proRes422` and `proRes4444` video codecs
+- `'mp4'` otherwise
+
+Values:
+
+- `'m4a'` is the only valid option for [audio](#recorderstartrecordingaudiooptions) recordings
+- `'mov'` is the only valid option for `proRes422` and `proRes4444` video codecs
+- `'mp4' | 'm4v' | 'mov'` for all other video codecs
+
+The extension of the output file
+
 #### recorder
 
 #### recorder.startRecordingScreen(options)

utils.js

@@ -80,6 +80,10 @@ export function normalizeOptions(targetType, {
 		recorderOptions.videoCodec = videoCodec;
 	}
 
+	if (targetType === 'audioOnly' && extension !== 'm4a') {
+		throw new Error('Audio recordings only supports the m4a extension');
+	}
+
 	const temporaryPath = temporaryFile({
 		extension: targetType === 'audioOnly' ? 'm4a' : extension,
 	});