[docs] LibraryEvolution: Point out a Central Tenet of Swift:

  The Default Behavior Should Be Safe

Also, remove the line about "providing tools to check changes", because
it no longer fits in the prologue flow and isn't core to the model.
The section on it is good enough.

Author: jrose-apple

docs/LibraryEvolution.rst

@@ -25,6 +25,9 @@ unacceptable cost or force widespread opting-out of abstraction. We intend to
 mitigate this primarily by designing the language and its implementation to
 minimize unnecessary and unintended abstraction:
 
+* Avoiding unnecessary language guarantees and taking advantage of that
+  flexibility to limit load-time costs.
+
 * Within the domain that defines an entity, all the details of its
   implementation are available.
 
@@ -35,10 +38,9 @@ minimize unnecessary and unintended abstraction:
   independently desirable to reduce accidental API surface area, but happens to
   also interact well with the performance design.
 
-* Avoiding unnecessary language guarantees and taking advantage of that
-  flexibility to limit load-time costs.
-
-We also intend to provide tools to detect inadvertent changes in interfaces.
+This last point is a specific case of a general tenet of Swift: **the default
+behavior is safe**. Where possible, choices made when an entity is first
+published should not limit its evolution in the future.
 
 .. contents:: :local: