[stdlib][NFC] Doc adjustments

Author: lorentey

stdlib/public/core/StringIndex.swift

@@ -49,27 +49,27 @@ isn't frozen.
 
 <resilience barrier>
 
-  * b3: UTF-16 encoding
+  * b0: `_isScalarAligned`
 
-    If set, the position is known to be expressed in UTF-16 code units.
+    If set, index is known to be on a Unicode scalar boundary (see below).
+    (Introduced in Swift 5.1)
+
+  * b1: `_isCharacterAligned`
+
+    If set, the index is known to be on an extended grapheme cluster
+    boundary (i.e., on a Swift `Character`.)
     (Introduced in Swift 5.7)
 
   * b2: UTF-8 encoding
 
     If set, the position is known to be expressed in UTF-8 code units.
     (Introduced in Swift 5.7)
 
-  * b1: `_isCharacterAligned`
+  * b3: UTF-16 encoding
 
-    If set, the index is known to be on an extended grapheme cluster
-    boundary (i.e., on a Swift `Character`.)
+    If set, the position is known to be expressed in UTF-16 code units.
     (Introduced in Swift 5.7)
 
-  * b0: `_isScalarAligned`
-
-    If set, index is known to be on a Unicode scalar boundary (see below).
-    (Introduced in Swift 5.1)
-
 Before Swift 5.7, bits b1, b2 and b3 used to be part of the resilient slice. See
 the notes on Character Alignment and Index Encoding below to see how this works.