Mention elemidx and dataidx in the Linking.md documentation

Hello,

I am writing a wasm backend for my compiler and I was planning to output relocatable wasm modules.

In the Linking.md document, there is no mention of dataidx and elemidx renumbering such that we would need to pad the LEB128 encodings of these to 5 bytes. I believe this should be new relocation types but these do not currently exist because LLVM does not support the element section or generate any memory.init or data.drop instruction.

I lost quite a bit of time wondering why the documentation did not address this issue. So here are my questions:

First, did I understood the problem properly and there is no mention of data or element segments reordering because these are not generated/supported by LLVM/lld?
Second, if this is correct, can I submit a PR updating the markdown documentation?

Thank you for your help. :)

Start with Linking.md and review the sections describing relocatable modules and index renumbering. Verify the questions about dataidx and elemidx against the documented WebAssembly conventions and LLVM/lld support mentioned in the issue. Done means resolving the ambiguity and updating the Markdown documentation if the omission is confirmed.