Clarify f.6 note on noexcept and allocation

The bigger picture issue of "What context is your code executing in?"
and "What type of code are the core guidelines addressing?" should be
handled separately with text in the overview.

CppCoreGuidelines.md

@@ -2144,9 +2144,24 @@ Unless the program is crafted to survive memory exhaustion, that may be just the
 
 ##### Note
 
-In most programs, most functions can throw
+You must be aware of the execution environment that your code is running when
-(e.g., because they use `new`, call functions that do, or use library functions that reports failure by throwing), so don't just sprinkle `noexcept` all over the place.
+deciding whether to tag a function `noexcept`, especially because of the issue
-`noexcept` is most useful for frequently used, low-level functions.
+of throwing and allocation.  Code that is intended to be perfectly general (like
+the standard library and other utility code of that sort) needs to support
+environments where a `bad_alloc` exception may be handled meaningfully.
+However, the majority of programs and execution environments cannot meaningfully
+handle a failure to allocate, and aborting the program is the cleanest and
+simplest response to an allocation failure in those cases.  If you know that
+your application code cannot respond to an allocation failure, it may be
+appropriate to add `noexcept` even on functions that allocate.
+
+Put another way: In most programs, most functions can throw (e.g., because they
+use `new`, call functions that do, or use library functions that reports failure
+by throwing), so don't just sprinkle `noexcept` all over the place without
+considering whether the possible exceptions can be handled.
+
+`noexcept` is most useful (and most clearly correct) for frequently used,
+low-level functions.
 
 ##### Note