nicer warning message for: Requested formats are incompatible for merge and will be merged into mkv

[JensKorte]

• I've verified and I assure that I'm running youtube-dl 2019.02.18
• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Other

---

Description of your issue, suggested solution and other information

The warning message "WARNING: Requested formats are incompatible for merge and will be merged into mkv." is not easy to understand when reading it for the first time: "are incompatible for merge and will be merged".

Could you please extend the warning to something like:

• WARNING: Requested formats are incompatible for direct merge and will be merged into mkv.
• WARNING: Requested formats are incompatible for merge in the video format/video container and will be merged into mkv.
• WARNING: Requested formats cannot be merged in one of themselfs themselves and will be merged into a mkv container.
• WARNING: Requested formats cannot be merged into one of themselfs themselves and will be merged into mkv.

(Edited after reading comment from rautamiekka)

[rautamiekka]

If WARNING: Requested formats are incompatible for merge in the video format/video container and will be merged into mkv. was to be chosen, I'd expand it to say merging into the specified video format/video container.

In the last 2 ones themselfs is a violation and should be themselves. In fact the in one of doesn't make sense, not sure if it's a violation.

[JensKorte]

If WARNING: Requested formats are incompatible for merge in the video format/video container and will be merged into mkv. was to be chosen, I'd expand it to say merging into the specified video format/video container.

I would prefer "selected" because it is easier, but I don't know which one is better.