docs: clarify flake types and implied defaults

[iFreilicht]

Motivation

What flake types are implied depending on context is currently not clear.

This PR fixes a few minor issue, makes it clear that the default flake type is indirect and clearly shows under which circumstances the types path: or git+file: can be implied.

Context

I wanted to fix path being incorrectly shown as the default type for a while, but this comment finally nudged me to do it.

Priorities

Add 👍 to pull requests you find important.

[thufschmitt]

Thanks, quite helpful indeed :)

[fricklerhandwerk]

Looks good to me, strict improvement. Also scary much complexity @roberth.

[fricklerhandwerk]

FYI: Following recent discussions it looks like we're going to call it just "registry" (as it's a convenience layer for fetching remote file system objects, which is not specific to flakes) and it may get constrained to be only available on command line calls.

In any case it would be good to link to documentation on the registry here. I see you just moved that text, but it may be worthwhile improving bits here and there while at it.

[iFreilicht]

Good point! I'd be happy to make another PR that replaces all mentions of the flake registry or nix registry with "registry" and link all those occurrences to nix3-registry.md.