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/lsp/protocol.lua

@@ -5,14 +5,14 @@ local if_nil = vim.F.if_nil
 local protocol = {}
 
 --[=[
---@private
+---@private
 --- Useful for interfacing with:
 --- https://github.com/microsoft/language-server-protocol/raw/gh-pages/_specifications/specification-3-14.md
 function transform_schema_comments()
   nvim.command [[silent! '<,'>g/\/\*\*\|\*\/\|^$/d]]
   nvim.command [[silent! '<,'>s/^\(\s*\) \* \=\(.*\)/\1--\2/]]
 end
---@private
+---@private
 function transform_schema_to_table()
   transform_schema_comments()
   nvim.command [[silent! '<,'>s/: \S\+//]]