ARROW-4244: [Format] Clarify padding/alignment rationale/recommendation.
ARROW-4244: [Format] Clarify padding/alignment rationale/recommendation.

I think this makes it the spec more consistent (but I might have misunderstood the intent).

Author: Micah Kornfield <emkornfield@gmail.com>
Author: emkornfield <emkornfield@gmail.com>

Closes #3388 from emkornfield/clarify_padding_spec and squashes the following commits:

10c74f13f <emkornfield> Add emphasis on preferred padding/alignment.
5cdde6f35 <Micah Kornfield> more clarification
454f06547 <Micah Kornfield> remove parenthetical that doesn't make sense anymore
aac1f8fb3 <Micah Kornfield> Clarify padding/alignment rationale.

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