Make abstracts one sentence long.

The second sentence was added while rewriting to remove 'iff' in commit
6bc4c85.

Author: amartini51

stdlib/public/Darwin/Foundation/NSStringAPI.swift

@@ -1669,7 +1669,7 @@ extension StringProtocol where Index == String.Index {
   // forward trivially to rangeOfString.
 
   /// Returns `true` if `other` is non-empty and contained within `self` by
-  /// case-sensitive, non-literal search. Otherwise, returns `false`.
+  /// case-sensitive, non-literal search; otherwise, returns `false`.
   ///
   /// Equivalent to `self.range(of: other) != nil`
   public func contains<T : StringProtocol>(_ other: T) -> Bool {

stdlib/public/core/ManagedBuffer.swift

@@ -376,7 +376,7 @@ extension ManagedBufferPointer {
   }
 
   /// Returns `true` if `self` holds the only strong reference to its
-  /// buffer. Otherwise, returns `false`.
+  /// buffer; otherwise, returns `false`.
   ///
   /// See `isKnownUniquelyReferenced` for details.
   @inlinable