Add spec

Author: odersky

docs/docs/reference/soft-modifier.md

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+---
+layout: doc-page
+title: Soft Modifiers
+---
+
+A soft modifier is one of the identifiers `opaque` and `inline`.
+
+It is treated as a potential modifier of a definition, if it is followed by a hard modifier or a keyword combination starting a definition (`def`, `val`, `var`, `type`, `class`, `case class`, `trait`, `object`, `case object`, `enum`). Between the two words there may be a sequence of newline tokens and soft modifiers.
+
+It is treated as a potential modifier of a parameter binding unless it is followed by `:`.
+