Provide description for additional mappings

[helins]

Is your feature request related to a problem? Please describe.
When adding mappings in the configuration of a picker, there does not seem to be a way for providing a description that would be rendered by which_key.

Describe the solution you'd like
Ideally, a description could be provided under desc in the opts of the mapping, to mimick the standard way.

Additional context
Without a description, which_key shows as a description some gibberish like _13_.

[Conni2461]

we kinda already support this but i think not consistently, or there is some bug. e.g. it doesnt work for key_func.type == "command" but it just works if you pass in a function: ["<c-c>"] = { function() vim.cmd [[ stopinsert ]] end, opts = { desc = "stopinsert" } },. at least on latest master, it does not work on 0.1.5 yet because it would be a new feature, so semver says we are not allowed to backport new features 😆

can you provide the additional mappings that you've added? because i think there should never be a _13_.

[ColinKennedy]

Came here to comment the same issue. Here's a not-so-minimal example

  pickers.new(
    opts,
    {
      prompt_title = "Session Viewer",
      finder = finders.new_table(
        {
          results = vim.tbl_flatten(sessions),
          entry_maker = function(entry)
            -- ... More stuff here ...
            return result
          end,
        }
      ),
      sorter = telescope_config.file_sorter(opts),
      attach_mappings = function(prompt_buffer, map)
        map(
          "n",
          "n",
          function() _USE_NICE_NAMES = not _USE_NICE_NAMES end,
          -- NOTE: `desc` is currently unsupported by telescope.nvim
          -- Reference: https://github.com/nvim-telescope/telescope.nvim/issues/2981
          --
          {desc = "Toggle [n]ice names or full path displays."}
        )

        map(
          "i",
          "<CR>",
          function()
            actions.close(prompt_buffer)

            local selection = state.get_selected_entry()

            -- Schedule buffers cleanup to avoid callback issues and source the session
            vim.schedule(
              function()
                local path = selection.value

                configuration.DATA.run_pre_switch_hook(path)

                vim.cmd.wall()
                vim.cmd[[%bwipeout]]
                vim.cmd.source(path)

                configuration.DATA.run_post_switch_hook(path)
              end
            )
          end,
          -- NOTE: `desc` is currently unsupported by telescope.nvim
          -- Reference: https://github.com/nvim-telescope/telescope.nvim/issues/2981
          --
          {desc = "Select a session to load."}
        )

        return true
      end,
    }):find()

When you press ? in the Telescope floating window, it renders ()_USE_NICE_NAMESnot_USE_NICE… rather than the intended desc