More prominently indicate that pb_backlog should be increased

It is a frequent customer issue to find that `pb_backlog` has not been
increased in a Riak CS installation. The additional wording should more
clearly indicate that the default, commented-out value actually sets it
to `5` and thus it should be increased.

Fixes #1033
Tweak thanks @lucperkins
Merge pull request #1068 from basho/issue-1033-pb_backlog-for-cs

More prominently indicate that pb_backlog should be increased. Thanks @joecaswell