docs: update completion guide

[alexandear]

This PR updates "Shell Completion" guide.

[ldez]

The current sentences are a note about a limitation with Bash, this is not Bash section, we will not describe each completion possibility (and only on macOS) it makes no sense.

[alexandear]

So, maybe we can delete Linux section also? And leave only:

---

Shell Completion

golangci-lint can generate Bash, fish, PowerShell, and Zsh completion files.

See the instructions on golangci-lint completion <YOUR_SHELL> --help (replace <YOUR_SHELL> with your favorite one).

macOS

There are two versions of bash-completion, v1 and v2. V1 is for Bash 3.2 (which is the default on macOS), and v2 is for Bash 4.1+. The golangci-lint completion script doesn’t work correctly with bash-completion v1 and Bash 3.2. It requires bash-completion v2 and Bash 4.1+. Thus, to be able to correctly use golangci-lint completion on macOS, you have to install and use Bash 4.1+ (instructions). The following instructions assume that you use Bash 4.1+ (that is, any Bash version of 4.1 or newer).

Install bash-completion v2:

brew install bash-completion@2
echo 'export BASH_COMPLETION_COMPAT_DIR="/usr/local/etc/bash_completion.d"' >>~/.bashrc
echo '[[ -r "/usr/local/etc/profile.d/bash_completion.sh" ]] && . "/usr/local/etc/profile.d/bash_completion.sh"' >>~/.bashrc
exec bash # reload and replace (if it was updated) shell
type _init_completion && echo "completion is OK" # verify that bash-completion v2 is correctly installed

Add golangci-lint bash completion:

echo 'source <(golangci-lint completion bash)' >>~/.bashrc
source ~/.bashrc

[ldez]

It's a better idea.

[ldez]

LGTM