To define a "Comments" field properly on a page in Business Central:

1. MultiLine = True: This property is required for text fields where users need to enter multiple lines of text, typically used for descriptions or comments.
2. ExtendedDatatype: This is the property name used to specify a specialized behavior or display format for a field (e.g., Phone No, Email, or RichContent). Note that "RichDataType" is not a valid AL property name.
3. RichContent: When used with ExtendedDatatype, this value enables the Rich Text Editor, allowing users to format text (bold, italics, etc.). TextRichContent is not a standard enum value for this property in AL.

Microsoft Learn: AL Language Reference - MultiLine Property. Link

Microsoft Learn: AL Language Reference - ExtendedDatatype Property. Link

Microsoft Learn: Working with Rich Text in Business Central. Link

Dynamics 365 Business Central Development: Table and Page Field Properties, Section: ExtendedDatatype options.