doc: specify that longDescription should be Markdown

[ncfavier]

See NixOS/nixos-search#525 for context.

In the spirit of RFC 72, document that longDescription is in Markdown format (should this be CommonMark?).

This is purely a documentation change because I don't think there are currently any consumers of longDescription that rely on it being a particular format besides plain text.

Producers, on the other hand, seem to have been using the Markdown convention, if any.

[fricklerhandwerk]

Please reference RFC72 in the commit message so we have a paper trail for the reasoning behind the change.
(I recommend writing the motivation in the commit message and re-using it for the pull-request description.)

[pennae]

maybe we should link to the nixpkgs chapter of markdown extensions instead, but using plain commonmark for long descriptions also also perfectly fine.

[roberth]

Suggested change

	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown format.
	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown.

Either use this suggestion, or add "the".

[roberth]

Or to follow pennae's suggestion: Nixpkgs Markdown.

[ncfavier]

I don't think we want to use any extensions in package descriptions, so CommonMark should be fine.

[SuperSandro2000]

This is purely a documentation change because I don't think there are currently any consumers of longDescription that rely on it being a particular format besides plain text.

Yes, currently it is plain text.

[SuperSandro2000]

Suggested change

	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown.
	An arbitrarily long description of the package in [CommonMark Markdown](https://commonmark.org).

[jtojnar]

That makes it look too much like there is a thing called CommonMark Markdown, maybe use something like the following instead:

Suggested change

	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown.
	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown dialect.

[roberth]

@jtojnar in that case, add "the".

Suggested change

	An arbitrarily long description of the package in [CommonMark](https://commonmark.org) Markdown.
	An arbitrarily long description of the package in the [CommonMark](https://commonmark.org) Markdown dialect.

[roberth]

My latest suggestion was 50% ironic and this is 100% my last suggestion.

[ncfavier]

I think the sentence is fine as it is and the bikeshed will stay purple.

[SuperSandro2000]

No, no, no, the bikeshed must be blue. Anything else is ridiculous.