deps: split and clarify documentation

`brew deps` has three different modes of operation that were not easy to
infer from the existing documentation. Split the help text into three
parts to make this clearer. This also improves on the confusion when
options were silently ignored because they don't apply to a certain mode
of operation. Those are:

1. List the shared dependencies (either intersection or union) of
   explicitly named formulae with options for traversal depth/order.

2. Output separate trees of explicitly listed or all installed formulae.

3. List all available/installed formulae and their direct dependencies
   with one line per formula formatted as `<formula>: <dependencies>`.

Closes #137.

Closes #179.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

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