Avoid repetition across slides

Author: LukeMathWalker

src/idiomatic/leveraging-the-type-system/newtype-pattern/is-it-encapsulated.md

@@ -4,7 +4,7 @@ minutes: 5
 
 # Is It Truly Encapsulated?
 
-You must evaluate the entire API surface exposed by a newtype to determine if
+You must evaluate _the entire API surface_ exposed by a newtype to determine if
 invariants are indeed bullet-proof. It is crucial to consider all possible
 interactions, including trait implementations, that may allow users to bypass
 validation checks.

src/idiomatic/leveraging-the-type-system/newtype-pattern/parse-don-t-validate.md

@@ -48,14 +48,6 @@ impl Username {
   underlying value since `&str`, the returned type, restricts them to read-only
   access.
 
-- Stress the importance of evaluating _the entire API surface_ exposed by a
-  newtype to determine if invariants are indeed bullet-proof.
-
-  It is crucial to consider all possible interactions, including trait
-  implementations, that may allow users to bypass the invariants. For example,
-  if the `Username` type implements the `DerefMut` trait, users can modify the
-  underlying string directly, bypassing the validation checks in `new`.
-
 - Type-level invariants have second-order benefits.
 
   The input is validated once, at the boundary, and the rest of the program can