Document the interpretation of compat ranges in registries

[timholy]

This is a start on #1190. @StefanKarpinski or others, feel free to modify at will.

[KristofferC]

This might get obsoleted by #1410 where we also have the same hyphen compat specifier as the registry uses.

[nickrobinson251]

Bump?

[timholy]

Presumably this can be closed due to https://julialang.github.io/Pkg.jl/dev/compatibility/#Hyphen-specifiers-1, right?

[nickrobinson251]

Do hyphen ranges in a Project.toml and a registry's Compat.toml have the exact same semantics?

And the exact same valid syntax? e.g.

"0.7-0.8"  # [0.7.0, 0.9.0)
"0.7-0"    # [0.7.0, 1.0.0)
"0.8.6-0"  # [0.8.6, 1.0.0)
"0.7-*"    # [0.7.0, ∞)

is this valid syntax for a Project.toml?

If yes to both, can we just document that?

But don't non-hyphen range have a different meaning?
E.g.

# F/Foo/Compat.toml

["0.18.2-0.19"]
Bar = "0.5.0"

means Foo version [0.18.2, 0.20) allows only the exact version 0.5.0 of dependency Bar (and not e.g. Bar v0.5.1)?

Caveat: I do not understand Registry syntax, so these are all questions. Personally I've only need to understand registry syntax when the registry has been manually edited and accidentally introduced an error (which i've then tried to debug, by trying to read the registry)