[com.google.fonts/check/italic_axis_last]: Check lacks rationale

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Observed behaviour

[ital-axis-not-las] check lacks rationale

Expected behaviour

The rationale of ital-axis-not-last should provide actual rationale, not simply a failure message.

The original issue #3669 seems to have a decent rationale, if a little brief. It would be even better if we could show evidence of whether it actually causes problems to have the Italic axis listed before custom axes, but I can imagine this being useful after reading at least the basic rationale.