Document default cmp mappings

[salcode]

    ['<Down>'] = {
      i = mapping.select_next_item({ behavior = types.cmp.SelectBehavior.Select }),
    },
    ['<Up>'] = {
      i = mapping.select_prev_item({ behavior = types.cmp.SelectBehavior.Select }),
    },
    ['<C-n>'] = {
      i = mapping.select_next_item({ behavior = types.cmp.SelectBehavior.Insert }),
    },
    ['<C-p>'] = {
      i = mapping.select_prev_item({ behavior = types.cmp.SelectBehavior.Insert }),
    },
    ['<C-y>'] = {
      i = mapping.confirm({ select = false }),
    },
    ['<C-e>'] = {
      i = mapping.abort(),
    },

hrsh7th/nvim-cmp lua/cmp/config/mapping.lua