Fix a bug in the generation of completion information

Completion information for multi-command binaries were not generated in
the correct way, so the build-bash-complation script failed when such a
binary was actually found.

This commit fixes this issue, and a related docstring typo as well.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>