Add new Documentation.Format kind for Signatures, make repeated and upstream

[Strum355]

Followed on from #131 (comment).

Additional context:
This will allow us to not have to hijack SymbolInformation.display_name in the LSIF Snapshotting, instead giving an alternative way to encode a signature, as a rendered string as opposed to a typed tree

[olafurpg]

@Strum355 should we close this issue as fixed? The Documentation.Format enum has been upstreamed, but it's not repeated (which doesn't seem to be necessary for lsif-java). We can always make it repeated in the future while keeping protobuf binary compatibility.