Clarify that automatic subtitles are generated.

It wasn't clear what automatic word mean.

README.md

@@ -329,8 +329,8 @@ which means you can modify it, redistribute it or use it however you like.
 
 ## Subtitle Options:
     --write-sub                      Write subtitle file
-    --write-auto-sub                 Write automatic subtitle file (YouTube
-                                     only)
+    --write-auto-sub                 Write automatically generated subtitle file
+                                     (YouTube only)
     --all-subs                       Download all the available subtitles of the
                                      video
     --list-subs                      List all available subtitles for the video

youtube_dl/YoutubeDL.py

@@ -156,7 +156,7 @@ class YoutubeDL(object):
     writethumbnail:    Write the thumbnail image to a file
     write_all_thumbnails:  Write all thumbnail formats to files
     writesubtitles:    Write the video subtitles to a file
-    writeautomaticsub: Write the automatic subtitles to a file
+    writeautomaticsub: Write the automatically generated subtitles to a file
     allsubtitles:      Downloads all the subtitles of the video
                        (requires writesubtitles or writeautomaticsub)
     listsubtitles:     Lists all available subtitles for the video

youtube_dl/options.py

@@ -363,7 +363,7 @@ def parseOpts(overrideArguments=None):
     subtitles.add_option(
         '--write-auto-sub', '--write-automatic-sub',
         action='store_true', dest='writeautomaticsub', default=False,
-        help='Write automatic subtitle file (YouTube only)')
+        help='Write automatically generated subtitle file (YouTube only)')
     subtitles.add_option(
         '--all-subs',
         action='store_true', dest='allsubtitles', default=False,