fix: add missing luasnip documentation

[soifou]

This fix an error when attempting to show the doc of a lua snippet, parsing the details fails because it expects a string not a table.

...re/nvim/lazy/blink.cmp/lua/blink/cmp/lib/window/docs.lua:239: attempt to call method 'gmatch'
(a nil value)

Also add the missing description

[Saghen]

Incorporated it directly into main with some changes, marked you as co-author. Thanks!

[soifou]

Yes your approach is better 👍 and thanks for the builtin module, really like it