Document JSON-RPC display protocol

I've begun looking into https://github.com/vshaxe/haxe-language-server to try to implement the new IDE features in a new Emacs mode, however I can't make heads or tails of how the JSON-RPC works.
Some additional documentation to https://haxe.org/manual/cr-completion-overview.html regarding its usage would be great.

Start by reading the Haxe completion overview at haxe.org/manual/cr-completion-overview.html and then trace the JSON-RPC behavior in the linked vshaxe/haxe-language-server. Document the protocol's usage for IDE features, including enough context for an Emacs mode, and update the manual page with the resulting explanation.

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