Expand style guide on naming

An attempt to capture the discussion at https://groups.google.com/d/msg/julia-dev/8ty9cQe_uGM/mkk3PiDG17wJ,
particularly https://groups.google.com/d/msg/julia-dev/8ty9cQe_uGM/Qgu9I6lN-LgJ
Merge pull request #8031 from JuliaLang/teh/styleguide

Expand style guide on naming