modules/lsp/onAttach: fix bufnr and document event arg

[MattSturgeon]

Fixes #3319

See :h event-args and :h LspAttach

I'm unsure whether we should consider this advice from the docs:

Note: If the LSP server performs dynamic registration, capabilities may be registered any time after LspAttach. In that case you may want to handle the "registerCapability" event.

Example:

vim.lsp.handlers['client/registerCapability'] = (function(overridden)
  return function(err, res, ctx)
    local result = overridden(err, res, ctx)
    local client = vim.lsp.get_client_by_id(ctx.client_id)
    if not client then
      return
    end
    -- Call your custom on_attach logic...
    -- my_on_attach(client, vim.api.nvim_get_current_buf())
    return result
  end
end)(vim.lsp.handlers['client/registerCapability'])

But that's out of scope for this PR. See #3323

[GaetanLepage]

LGTM, thanks

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