[pruned-liveness] Fix incorrect header comment that didn't describe the algorithm correctly.

Author: gottesmm

include/swift/SIL/PrunedLiveness.h

@@ -57,21 +57,21 @@
 ///
 /// Example 2. Cross-block liveness.
 ///
-/// Initial State:
+/// Initial State after initializing a def block:
 ///
 ///  -----
-/// | Def | [LiveOut]
+/// | Def | [LiveWithin]
 ///  -----
 ///    |
 ///  -----
 /// |     | [Dead]
 ///  -----
 ///    |
 ///  -----
-/// |     | [Dead]
+/// | Use | [Dead]
 ///  -----
 ///
-/// State after updateForUse:
+/// Later state after updateForUse is applied to the use:
 ///
 ///  -----
 /// | Def | [LiveOut]
@@ -85,6 +85,10 @@
 /// | Use | [LiveWithin]
 ///  -----
 ///
+///
+/// An invariant is that for any liveness region, the post-dominating blocks of
+/// the region are the LiveWithin regions.
+///
 //===----------------------------------------------------------------------===//
 
 #ifndef SWIFT_SILOPTIMIZER_UTILS_PRUNEDLIVENESS_H