[intro.progress] Fixup for 'move implementation recommendations to outside notes'

Author: jensmaurer

source/basic.tex

@@ -6234,12 +6234,20 @@
   When one or more lock-free executions run concurrently,
   at least one should complete.
   \begin{note}
-    That means programmers can safely ignore anomalies
-    caused by repeated and particularly inopportune interference
-    from other threads, for example by repeatedly stealing a cache line
+    It is difficult for some implementations
+    to provide absolute guarantees to this effect,
+    since repeated and particularly inopportune interference
+    from other threads
+    could prevent forward progress,
+    e.g.,
+    by repeatedly stealing a cache line
     for unrelated purposes
-    between load-locked and store-conditional instructions,
-    which could otherwise indefinitely delay progress.
+    between load-locked and store-conditional instructions.
+    For implementations that follow this recommendation and
+    ensure that such effects cannot indefinitely delay progress
+    under expected operating conditions,
+    such anomalies
+    can therefore safely be ignored by programmers.
     Outside this document,
     this property is sometimes termed lock-free.
   \end{note}