Clarify nightly style policy

Author: compiler-errors

nightly-style-policy.md

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+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. Initial PR implementors are 
+encouraged but not required to open a PR against 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, changes should
+be not 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 bheavior. At this point, T-style may also propose alternative
+formatting at the time of stabilization, with any breaking changes weighted
+according to the principle stated above.