Document _prefix index matching for queries

[dnephin]

This behaviour is documented on an unexported function, but it doesn't show up in the godoc. I've attempted to move it to all the exported functions that call getIndexValue

[radeksimko]

Do you mind rebasing / resolving conflicts? I think this would be a nice addition otherwise.

---

On a related note I've been thinking about adding a simple memdb.Prefix(string) function which would avoid the need for "special strings" and make navigation and search through codebase with prefixed indexes easier. However it doesn't look very appealing from the perspective of character length vs just simple _prefix appended to the name. :|

I just feel that this should be somehow more prominent part of the API. I didn't even realize it existed before reading through Nomad/Consul codebases and realizing _prefix actually has a special meaning.

[dnephin]

Thanks for having a look! I've rebased this PR.

I agree with you about exposing it in some way that makes it more obvious. I was also thinking that memdb.Prefix(string) would be nice.