tweak 'brew deps' zsh completions

[apjanke]

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This tweaks the zsh completions for brew deps.

• Fixes a couple typos
• Removes reference to "dependents" from description, since only dependencies are displayed (dependents are displayed by brew uses)
• Harmonize description text for --installed to reduce redundant display
• Note the two-behavior nature of --installed when used with regular formula display
• Add description for --tree
• Reflect that --tree does not respect --include-* or --skip-recommended

[MikeMcQuaid]

Great work. Crazy long-term stretch-goal: generate completions from --help output.

[apjanke]

Thanks!

Crazy long-term stretch-goal: generate completions from --help output.

Too crazy for me, I think. Given the interactions between options (e.g. if --foo is present, do not suggest completion of --bar or this other set of options...) you'd need to embed equivalent metadata in a variant --help output. Or switch to a programmatic option definition, and generate both the --help and the completions from it. Or something. :)