scripts: update autocomplete instructions

[efiop]

We are now including bash and zsh completion scripts into our deb, rpm and brew packages. So users of those don't need to get the scripts by-hand.

[ptisserand]

Hi, do you mean it's an update of docs/install/completion.md to use installed completion scripts ?

[dashohoxha]

@ptisserand I think that you should first understand the different modes of installation, and even try out some of them if possible. You should also familiarize yourself with the installation of tab completion.
Only after that you might be able to modify properly the installation instructions on that page.

But maybe someone else can explain you better, I am not sure.

[shcheklein]

@ptisserand yes, this is an update to the completion.md to mention that in certain cases completion is enabled out of the box. I would just write a simple sentence before each section - Ubuntu/Mac/etc ...

[jorgeorpinel]

@ptisserand thanks for your interest! On second thought this was not such a great first issue though.

[jorgeorpinel]

I'm addressing this in a0cb947.