Minor wording tweaks

[macintux]

• Avoid ambiguity on the non-singular nature of query buffer managers
• Clarify when query buffers are used

References code changes in https://github.com/basho/riak_kv/pull/1587

/cc @hmmr

[macintux]

@laurenrother What's up with this failure?

[lrnrthr]

@macintux The Travis failure? It's failing b/c it wouldn't be able to build the docs w/out the download packages. :) Which... we would expect for a version not yet out. We've opted, short-term, to let the failure stand and consciously ignore it rather than having it ignore downloads and possibly miss a problem

[macintux]

Closing in favor of #2424