[gardening] Add comments explaining what alloc_stack [dynamic_lifetime] means.

Found the meaning by looking at when Definite Initialization marks alloc_stack
with that marker.

For those who are unware like me, it means the liveness of the value in the
alloc_stack is conditional in some way (i.e. conditional init, destroy, etc).

Author: gottesmm

include/swift/SIL/SILInstruction.h

@@ -1444,7 +1444,18 @@ class AllocStackInst final
     }
   }
 
+  /// Set to true that this alloc_stack contains a value whose lifetime can not
+  /// be ascertained from uses.
+  ///
+  /// As an example if an alloc_stack is known to be only conditionally
+  /// initialized.
   void setDynamicLifetime() { dynamicLifetime = true; }
+
+  /// Returns true if the alloc_stack's initialization can not be ascertained
+  /// from uses directly (so should be treated conservatively).
+  ///
+  /// An example of an alloc_stack with dynamic lifetime is an alloc_stack that
+  /// is conditionally initialized.
   bool hasDynamicLifetime() const { return dynamicLifetime; }
 
   /// Return the debug variable information attached to this instruction.