Add "take a copy for immediate local use" case to F.16 (#2172)

Closes #2170

CppCoreGuidelines.md

@@ -3007,7 +3007,8 @@ When copying is cheap, nothing beats the simplicity and safety of copying, and f
 For advanced uses (only), where you really need to optimize for rvalues passed to "input-only" parameters:
 
 * If the function is going to unconditionally move from the argument, take it by `&&`. See [F.18](#Rf-consume).
-* If the function is going to keep a copy of the argument, in addition to passing by `const&` (for lvalues),
+* If the function is going to keep a locally modifiable copy of the argument only for its own local use, taking it by value is fine
+* If the function is going to keep a copy of the argument to pass to another destination (to another function, or store in a non-local location), in addition to passing by `const&` (for lvalues),
   add an overload that passes the parameter by `&&` (for rvalues) and in the body `std::move`s it to its destination. Essentially this overloads a "will-move-from"; see [F.18](#Rf-consume).
 * In special cases, such as multiple "input + copy" parameters, consider using perfect forwarding. See [F.19](#Rf-forward).