[css-sizing-3][css-flexbox-1][css-grid-1] Tighten up sizing terminology. #2385

Need to audit “behaves as auto” (a predicate which should probably be replaced by the noun “automatic size”), “automatic size”, “automatic minimum size”, “minimum size”, and “maximum size” to make sure they're all used correctly and consistently, and properly cross-linked (now that we have anchors for them in Sizing).