Add more information on overriding providers

As noted in IRC, if you are using a custom linux spin or derivative, it
may not be apparent which provider to use when overriding providers in
the minion config file. This commit adds a massive table for the pkg,
service, user, and group providers which explains how they are used, to
help guide the user to the correct decision when specifying a provider
override.
Fix sphinx build warning
Merge pull request #6844 from terminalmage/docs

Add more information on overriding providers