[docs] Clarify `best*`

Closes #4373

README.md

@@ -1443,7 +1443,7 @@ You can also use special names to select particular edge case formats:
 
  - `all`: Select **all formats** separately
  - `mergeall`: Select and **merge all formats** (Must be used with `--audio-multistreams`, `--video-multistreams` or both)
- - `b*`, `best*`: Select the best quality format that **contains either** a video or an audio
+ - `b*`, `best*`: Select the best quality format that **contains either** a video or an audio or both (ie; `vcodec!=none or acodec!=none`)
  - `b`, `best`: Select the best quality format that **contains both** video and audio. Equivalent to `best*[vcodec!=none][acodec!=none]`
  - `bv`, `bestvideo`: Select the best quality **video-only** format. Equivalent to `best*[acodec=none]`
  - `bv*`, `bestvideo*`: Select the best quality format that **contains video**. It may also contain audio. Equivalent to `best*[vcodec!=none]`