Explain a bit more on how to configure language server in Eglot's manual

[Yuan Fu]

Recently I was trying to use eglot with rust-analyzer and configure 
rust-analyzer. It turns out more confusing than it should be, and I think we 
can add a paragraph to eglot’s manual to help others like me.

LSP servers generally allow configuration through either LSP’s 
workspaceConfiguration protocol or their custom configuration file. For those 
who use workspaceConfiguration, it is common for the documentation to say 
something like this:

    This is the list of config options rust-analyzer supports:

    rust-analyzer.assist.emitMustUse (default: false) blah blah blah
    …

To pass this configuration to rust-analyzer through eglot, you need to set 
eglot-workspace-configuration to 

'(:rust-analyzer (:assist (:emitMustUse t)))

However, it is not obviously clear how to interpret 
“rust-analyzer.assist.emitMustUse”, not for someone inexperience with LSP, at 
least. (OTOH, the manual explains very clearly how to translate a JSON config 
to a plist, great job!)

I suggest we add a sentence or a paragraph to the eglot manual explaining that 
rust-analyzer.assist.emitMustUse translates to the JSON configuration

{
  "rust-analyzer": {
    "assist": {
      "emitMustUse": true
    }
  }
}

And the user should translate that to the plist format for 
eglot-workspace-configuration.

This is not something only rust-analyzer does. I surveyed a bunch of language 
servers, all the servers that support LSP’s workspaceConfiguration (typescript, 
rust, python, OCaml, Haskell, perl, Java) describe their configuration in the 
xxx.xx.xxx format. Because this is the format one can copy and paste into 
VSCode’s setting file.

We should probably also mention that the “root key” (the "rust-analyzer” part) 
isn’t always the name of the language server. Some server expects the language 
name rather than the server’s name, ie, haskell rather than 
haskell-language-server. Typescript expects “typescript” or “javascript”, perl, 
ocaml, java expects language names, python expects “pyls”, the name of the 
server.

Yuan