docs: account for bash-completion in suggested script

[EricFromCanada]

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If the bash-completion formula is installed, the completions it adds to /usr/local/etc/bash_completion.d can only be read via its bash_completion.sh script, and will error out due to missing functions if read directly. This changes the suggested script so it runs bash_completion.sh if available, and loads each file in /usr/local/etc/bash_completion.d otherwise (e.g. completions for brew and brew-services).

Note that for anyone with bash-completion installed and the suggested line from its Caveats added to their ~/.bash_profile, this script is unneccessary.

[MikeMcQuaid]

Nice work as usual!