Description in tag panel

I would like to a have a short description for tags added in the tag panel. For example, I added a SUBTITLE tag in the panel, and I would like to add a description "Ex: Op. 4" to explain how to use the tag.

You may edit the name of the tag panel field. e. g. ...
Field: SUBTITLE
Name: Subtitle (Ex: Op. 4)

DD.20180430.1115.CEST

I'm just curious:
How does "Ex: Op. 4" helps to understand why this should be a TIT3 (subtitle) content and not a TIT1 or TIT2 (title) content?
The official description for TIT3 says:

The 'Subtitle/Description refinement' frame is used for information directly related to the contents title (e.g. "Op. 16" or "Performed live at Wembley").

"Ex: Op. 4" was just for me to remind that I have to set this field to an opus number.
But the description could also be set to "The 'Subtitle/Description refinement' frame is used for information directly related to the contents title (e.g. "Op. 16" or "Performed live at Wembley")."
In that case, it may be too long to set it in the field title, it was for this reason that I think a description below the field value will be great to have a full multiline description.