Add notes on fixing "Connecting to kernel" failures.

[jd-foster]

This adds to the docs on the issue of fixing "Connecting to kernel" failures. This questions seems to arise often among a variety of users and may be worth providing guidance on how to address it.

For reference, here is a list of times where this issue has arisen, played a role or could be helped by a better understanding here: https://github.com/JuliaLang/IJulia.jl/issues/975 https://github.com/JuliaLang/IJulia.jl/issues/1001 https://github.com/JuliaLang/IJulia.jl/issues/1002 https://github.com/JuliaLang/IJulia.jl/issues/1021

and on Discourse: https://discourse.julialang.org/t/ijulia-kernel-not-starting-unless-ijulia-is-installed-in-the-global-environment-ie-v1-1/21172/4 https://discourse.julialang.org/t/launch-jupyter-kernel-with-e-using-somepackage-flag/47980