Clarify nightly style policy

nightly-style-procedure.md

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+The style guide may or may not prescribe formatting behavior for nightly syntax
+in the "nightly" chapter in preparation for a feature's eventual stabilization
+or to reflect actual the behavior implemented by rustfmt around nightly syntax.
+This behavior is not binding until the feature is stabilized and the text is moved
+into the appropriate section of the style guide.
+
+Initial PR(s) implementing new syntax filed against rust-lang/rust should
+generally leave the rustfmt subtree untouched. In cases that that need to
+modify rustfmt (for example, to fix compiler errors that come from adding
+new variants to AST representation), they should modify rustfmt in such a
+way to keep existing formatting verbatim.
+
+Rustfmt is allowed to implement nightly-only formatting behavior without that
+syntax being specified by the style guide. The initial authors of PRs
+implementing new features in rust-lang/rust are encouraged, but not
+required, to open a PR against
+[rustfmt](https://github.com/rust-lang/rustfmt) suggesting an initial
+formatting behavior, or formatting may later be implemented as a PR by anyone,
+pending approval of the implementation from T-rustfmt. T-style should be
+notified to approve the interim style proposed by these PRs, but this decision
+is not binding and may be revisited until the feature is stabilized and the
+formatting is codified in the style guide. 
+
+Much like breaking nightly feature changes in the Rust compiler, any changes
+to formatting behavior for nightly syntax should be made cautiously and with
+thorough consideration to avoid churn. Changes should not be done unnecessarily,
+and should take into account the feature's adoption
+and readiness for stabilization. However, changes may be done until the feature
+is stabilized for various reasons: new understanding of the feature's usage in
+the language, recommendation from T-style, or changes in the implementation of
+the feature.
+
+Feature stabilization should be blocked on confirmation and codification of
+formatting behavior. At this point, T-style may also propose alternative
+formatting at the time of stabilization, with any breaking changes weighted
+according to the breaking changes principle stated above.