docs(lua): add luv (vim.loop) reference manual (#19679)

Upstreamed from https://github.com/nanotee/luv-vimdocs with kind
permission from @nanotee.

runtime/doc/help.txt

@@ -187,6 +187,7 @@ Other ~
 |dev_style.txt|		Nvim style guide
 |job_control.txt|	Spawn and control multiple processes
 |luaref.txt|		Lua reference manual
+|luvref.txt|		Luv (|vim.loop|) reference manual
 
 						*standard-plugin-list*
 Standard plugins ~

runtime/doc/lua.txt

@@ -516,8 +516,8 @@ management. Try this command to see available functions: >
 
     :lua print(vim.inspect(vim.loop))
 <
-Reference: https://github.com/luvit/luv/blob/master/docs.md
-Examples:  https://github.com/luvit/luv/tree/master/examples
+Internally, `vim.loop` wraps the "luv" Lua bindings for the LibUV library;
+see |luv-intro| for a full reference manual.
 
                                                     *E5560* *lua-loop-callbacks*
 It is an error to directly invoke `vim.api` functions (except |api-fast|) in