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/snippet.lua

@@ -101,7 +101,7 @@ local function get_extmark_range(bufnr, extmark_id)
   return { mark[1], mark[2], mark[3].end_row, mark[3].end_col }
 end
 
---- @class vim.snippet.Tabstop
+--- @class (private) vim.snippet.Tabstop
 --- @field extmark_id integer
 --- @field bufnr integer
 --- @field index integer
@@ -177,7 +177,7 @@ function Tabstop:set_right_gravity(right_gravity)
   })
 end
 
---- @class vim.snippet.Session
+--- @class (private) vim.snippet.Session
 --- @field bufnr integer
 --- @field extmark_id integer
 --- @field tabstops table<integer, vim.snippet.Tabstop[]>