[rbi] Make a confusing comment clearer.

From talking with @dgregor, it became clear that this comment was easily
interpreted as saying that AssignFresh always introduced a disconnected value...
which is not the case. Instead, AssignFresh just introduces a new value that
could have any form of isolation. The actual isolation of the value is assigned
via tryToTrackValue and eventually SILIsolationInfo::get().

Author: gottesmm

include/swift/SILOptimizer/Utils/PartitionUtils.h

@@ -428,6 +428,10 @@ enum class PartitionOpKind : uint8_t {
   Assign,
 
   /// Assign one value to a fresh region, takes one arg.
+  ///
+  /// NOTE: This just produces a new value that is tracked by the dataflow. The
+  /// isolation characteristics of the value are actually decided by
+  /// tryToTrackValue and SILIsolationInfo::get().
   AssignFresh,
 
   /// Merge the regions of two values, takes two args, both must be from

lib/SILOptimizer/Analysis/RegionAnalysis.cpp

@@ -1553,8 +1553,13 @@ enum class TranslationSemantics {
   Ignored,
 
   /// An instruction whose result produces a completely new region. E.x.:
-  /// alloc_box, alloc_pack, key_path. This results in the translator
-  /// producing a partition op that introduces the new region.
+  /// alloc_box, alloc_pack, key_path, a function without any parameters. This
+  /// results in the translator producing a partition op that introduces the new
+  /// region.
+  ///
+  /// NOTE: This just introduces a new value and must always occur before a
+  /// value is actually used. The isolation of the value is actually determined
+  /// by invoking tryToTrackValue and SILIsolationInfo::get().
   AssignFresh,
 
   /// An instruction that merges together all of its operands regions and