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/iter.lua

@@ -64,11 +64,13 @@
 --- In addition to the |vim.iter()| function, the |vim.iter| module provides
 --- convenience functions like |vim.iter.filter()| and |vim.iter.totable()|.
 
+---@nodoc
 ---@class IterMod
 ---@operator call:Iter
 
 local M = {}
 
+---@nodoc
 ---@class Iter
 local Iter = {}
 Iter.__index = Iter
@@ -77,6 +79,7 @@ Iter.__call = function(self)
 end
 
 --- Special case implementations for iterators on list tables.
+---@nodoc
 ---@class ListIter : Iter
 ---@field _table table Underlying table data
 ---@field _head number Index to the front of a table iterator