pubsub: clarify doc for ReceiveSettings.NumGoroutines
pubsub: clarify doc for ReceiveSettings.NumGoroutines

Fixes #712.

Change-Id: I0c28081933c35ed3f2baf1617f39a2f92c7fd552
Reviewed-on: https://code-review.googlesource.com/15690
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go