Changed some of the wording in units docs to clarify what "unit formats" is about
added more subsections and generified base formats.rst title
Changed format.rst section title to @mdboom's suggestion

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Changed section title form "Units" to "Units and Quantities"

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Changed title of first subsection
Merge pull request #1707 from eteq/unit-formats-doc-clarification

Changed some of the wording in units docs to clarify what "unit formats" is about