Clarify reduction with empty axes and scalar input

[inexorabletash]

Improve the description of the reduction ops:

• Give in-line definition for L1/L2.
• Make "input values" slightly clearer.
• Explicitly note that if the input is a scalar, so is the output.
• State how the reduction function is applied for empty axes.
• Standardize on "reduction operation" not "reduce operation".
• Don't style list of reduction ops as non-normative note.

Fixes #740

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[inexorabletash]

@philloooo - does this make the behavior clearer?