clarify pointer add/sub function safety concerns

Ralf Jung made the same changes to the offset functions' documentation
in commit fb08915. As add/sub just call offset, the same limitation
applies here, as well.

Removed emphasis on review request by @joshtriplett

Author: jannic

src/libcore/ptr.rs

@@ -1255,7 +1255,7 @@ impl<T: ?Sized> *const T {
     /// Behavior:
     ///
     /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of an allocated object.
+    ///   byte past the end of the same allocated object.
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -1312,7 +1312,7 @@ impl<T: ?Sized> *const T {
     /// Behavior:
     ///
     /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of an allocated object.
+    ///   byte past the end of the same allocated object.
     ///
     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
     ///
@@ -1893,7 +1893,7 @@ impl<T: ?Sized> *mut T {
     /// Behavior:
     ///
     /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of an allocated object.
+    ///   byte past the end of the same allocated object.
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -1950,7 +1950,7 @@ impl<T: ?Sized> *mut T {
     /// Behavior:
     ///
     /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of an allocated object.
+    ///   byte past the end of the same allocated object.
     ///
     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
     ///