refactor(lua): improve type annotations

runtime/lua/vim/_meta/builtin.lua

@@ -121,6 +121,7 @@ function vim.stricmp(a, b) end
 --- @param str string
 --- @param index integer
 --- @param use_utf16? boolean
+--- @return integer
 function vim.str_byteindex(str, index, use_utf16) end
 
 --- Gets a list of the starting byte positions of each UTF-8 codepoint in the given string.

runtime/lua/vim/_meta/builtin_types.lua

@@ -34,6 +34,11 @@
 --- @field filename? string
 --- @field lnum integer
 
+--- @class vim.fn.getmarklist.ret.item
+--- @field mark string
+--- @field pos [integer, integer, integer, integer]
+--- @field file string
+
 --- @class vim.fn.getmousepos.ret
 --- @field screenrow integer
 --- @field screencol integer

runtime/lua/vim/_meta/vimfn.lua

@@ -3249,8 +3249,8 @@ function vim.fn.getloclist(nr, what) end
 --- Refer to |getpos()| for getting information about a specific
 --- mark.
 ---
---- @param buf? any
---- @return any
+--- @param buf? integer?
+--- @return vim.fn.getmarklist.ret.item[]
 function vim.fn.getmarklist(buf) end
 
 --- Returns a |List| with all matches previously defined for the