docs/developer-setup: Update documentation about auto-completion

Improving the postactivate hook example to work both for zsh/bash users

Test Plan

After amending the postactivate hook described in the diff:

$ bash
$ workon swh
$ swh-scheduler <tab> <tab> # <- completion should work and show the scheduler's cli subcommands
$ deactivate
$ zsh
$ workon swh  # <- this no longer displays warnings/errors about completion
$ swh-scheduler <tab> <tab>  # <- completion should now work as in the bash shell

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Migrated from D1131 (view on Phabricator)