docs: make Lua docstrings consistent #15255

The official developer documentation in in :h dev-lua-doc specifies to
use "--@" for special/magic tokens. However, this format is not
consistent with EmmyLua notation (used by some Lua language servers) nor
with the C version of the magic docstring tokens which use three comment
characters.

Further, the code base is currently split between usage of "--@",
"---@", and "--- @". In an effort to remain consistent, change all Lua
magic tokens to use "---@" and update the developer documentation
accordingly.

runtime/lua/vim/F.lua

@@ -2,8 +2,8 @@ local F = {}
 
 --- Returns {a} if it is not nil, otherwise returns {b}.
 ---
---@param a
---@param b
+---@param a
+---@param b
 function F.if_nil(a, b)
   if a == nil then return b end
   return a