Overhaul shell completion docs

[twz123]

Description

Fixes:

• Improve documentation for Zsh autocompletion #781

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[kke]

how about something like this?

grep -q .bash_completion.d .bashrc || cat <<'EOF'

[twz123]

I tried to find some references about the idiomatic way of including user-managed bash shell completions, but I didn't really find anything. The bash_completion.d stuff seems to be used, but there doesn't seem to be "the way of doing it". That's why I tried to give an example with the caveat "adjust to your personal needs". Checking if the folder has already been set up in .bashrc or some other place is not a piece of cake. That's why I opted for not doing it.

If you think checking would be better, then I'd probably change this to some descriptive block where to put the snippet, similar to the examples for zsh.

WDYT?