Add doc link to sync.WaitGroup in the package documentation.

Rationale:
The reference to sync.WaitGroup might be obvious to the authors of this
package, but it wasn't to me, so despites I had seen this package
multiple times I had never made the link to sync.WaitGroup. So when
playing with WaitGroup I was missing remembering about errgroup.
In hope this will help others as well as my future self.

Change-Id: I2c56976958ad9ab94c1596c96fa86c00dd73d2fb
Reviewed-on: https://go-review.googlesource.com/c/sync/+/538335
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>