Recommend to never display zero disambiguators when demangling v0 symbols

[michaelwoerister]

This PR extends the v0 symbol mangling documentation with the strong recommendation that demanglers should never display zero-disambiguators, especially when dealing with crate-root.

Being able to rely on C3foo to be rendered as foo (i.e. without explicit disambiguator value) rather than as foo[0] allows the compiler to encode things like new basic types in a backward compatible way. This idea has been originally proposed by @eddyb in the discussion around supporting f16 and f128 in the v0 mangling scheme. It is a generally useful mechanism for supporting a certain class of new elements in the v0 mangling scheme in a backward compatible way (whether as a temporary workaround until downstream tooling has picked up grammar changes or as a permanent encoding).

cc @tgross35

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[michaelwoerister]

cc @ehuss, as the original author of these (incredible) docs.

[GuillaumeGomez]

r? eddyb

[michaelwoerister]

@eddyb, would you like me to make any changes to this PR before it can be approved?