Document using modifiers to use same type of output format with variants

[cderv]

Is this a new currently undocumented feature or is this described somewhere in the documentation?

I would say this is an undocumented feature for now. We do handle specifically unknown Pandoc's extension as modifiers to differentiate format

quarto-cli/src/core/pandoc/pandoc-formats.ts

Lines 124 to 134 in 5eb78d9

	// Format strings are of the form:
	// baseName+<variants | modifiers>-<variants>[<extension>]
	// Where modifiers and variants can be in any order
	// and basename must start the string and extension must end the string
	export const parseFormatString = (formatStr: string): FormatDescriptor => {
	// acm-pdf+foo
	// gfm-raw_html
	// acm-2023-pdf+foobar
	// pdf
	// acm
	// acm-gfm-raw_html <-- not allowed

It is related to this one that works now better on 1.4

• Parsing format string with Pandoc's extensions  #4080

as reference from past discussion to document better

• Beamer: Render classoption: "handout, presentation" simultaneously #7218 (comment)
• Multiple pdf files #1473 (comment)

We should probably document this better indeed.

Originally posted by @cderv in #7325 (comment)