I have distilled the documentation to this:
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;;; Mtext formatting information:
;;;
;;; See: http://help.autodesk.com/view/ACD/2018/ENU/?guid=GUID-7D8BB40F-5C4E-4AE5-BD75-9ED7112E5967
;;;
;;; "\\\\" Escape sequence for \
;;; "\\{" {
;;; "\\}" }
;;;
;;; Codes without attributes and without a text sequence:
;;; "\\~" Non-breaking space.
;;; "\\P" Paragraph.
;;;
;;; Codes without attributes and with a text sequence ending in ";": "\\S1/2;"
;;; Code letter can be upper case or lower case.
;;; "\\S" Stacking. Stacking happens at the next "/", "#" or "^" symbol.
;;;
;;; Codes without attributes and with or without a bracketed text sequence: "{\\LText}"
;;; or an alternative text sequence: "\\OText\\o"
;;; Code letter can be upper case (= start of sequence) or lower case (= end of sequence).
;;; "\\K" Strikethrough. Not supported in BricsCAD: is displayed but mtext cannot be edited.
;;; "\\L" Underscore.
;;; "\\O" Overscore.
;;;
;;; Codes with attributes and without a text sequence:
;;; "\\A...; Alignment. Not supported in BricsCAD.
;;; "\\p...;" Bullets, numbered paragraphs and columns (tabs).
;;;
;;; Codes with attributes and with or without a bracketed text sequence: "{\\C1;Text}"
;;; Code letter can be upper or lower case.
;;; "\\C...;" Color. "\\C" for ACI color; "\\c" for true color.
;;; "\\F...;" Font.
;;; "\\H..x;" Height. The "x" is optional and indicates relative height.
;;; "\\Q...;" Obliquing (degrees).
;;; "\\T...;" Tracking, character spacing (factor).
;;; "\\W...;" Width, character width (factor).
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