docs: improve/add documentation of Lua types

- Added `@inlinedoc` so single use Lua types can be inlined into the
  functions docs. E.g.

  ```lua
  --- @class myopts
  --- @inlinedoc
  ---
  --- Documentation for some field
  --- @field somefield integer

  --- @param opts myOpts
  function foo(opts)
  end
  ```

  Will be rendered as

  ```
  foo(opts)

    Parameters:
      - {opts} (table) Object with the fields:
               - somefield (integer) Documentation
                 for some field
  ```

- Marked many classes with with `@nodoc` or `(private)`.
  We can eventually introduce these when we want to.

runtime/lua/vim/lsp/diagnostic.lua

@@ -390,7 +390,7 @@ local function clear(bufnr)
   end
 end
 
----@class lsp.diagnostic.bufstate
+---@class (private) lsp.diagnostic.bufstate
 ---@field enabled boolean Whether inlay hints are enabled for this buffer
 ---@type table<integer, lsp.diagnostic.bufstate>
 local bufstates = {}