update unicode section in toc.md

Author: jangko

docs/toc.md

@@ -89,13 +89,14 @@ can bring down the service, both lexer and parser are configurable to mitigate t
   - Input string:
     - Accepted encoding for input string are UTF-8.
     - Escaped unicode in quoted string take the form of UTF-16 BE:
-      - Fixed 4 digit hex: e.g. `\u000A`
-      - variable length: `\u{1F4A9}` with range (>= 0x0000 and <= 0xD7FF or >= 0xE000 and <= 0x10FFFF)
+      - Fixed length notation using 4 digit hex: e.g. `\u000A`.
+      - Variable length notation using curly braces `\u{1F4A9}` with range (0x0000..0xD7FF, 0xE000..0x10FFFF).
       - Escape sequences are only meaningful within a single-quoted string.
-        In multiline string, unicode char must be encoded using UTF-8.
-      - SurrogatePair: "\uD83D\uDCA9" is equal to "\u{1F4A9}"
+      - In multiline string, unicode char must be encoded using UTF-8.
+      - Surrogate pair using fixed length notation "\uD83D\uDCA9" is equal to variable length notation "\u{1F4A9}".
+      - Orphaned surrogate will result in error.
 
   - Output string:
     - Output string subject to output serialization format specification.
     - For example, output using json as serialization format will result in UTF-8 encoded string.
-    - Or if the escape flag is set, it will use UTF-16 BE 4 digit hex fixed length similar to GraphQL escape sequence.
+    - If the escape flag is set, it will use UTF-16 BE 4 digit hex fixed length similar to GraphQL escape sequence.