Fix regex doc [ci skip]

doc/schema.md

@@ -180,7 +180,7 @@ Besides, the `format` schema keyword for string values is ignored, since it is n
 
 The schema keyword `pattern` and `patternProperties` uses regular expression to match the required pattern.
 
-RapidJSON implemented a simple DFA regular expression engine, which is used by default. It supports the following syntax.
+RapidJSON implemented a simple NFA regular expression engine, which is used by default. It supports the following syntax.
 
 |Syntax|Description|
 |------|-----------|

doc/schema.zh-cn.md

@@ -180,7 +180,7 @@ RapidJSON 通过了 [JSON Schema Test Suite](https://github.com/json-schema/JSON
 
 `pattern` 及 `patternProperties` 这两个 schema 关键字使用了正则表达式去匹配所需的模式。
 
-RapidJSON 实现了一个简单的 DFA 正则表达式引擎，并预设使用。它支持以下语法。
+RapidJSON 实现了一个简单的 NFA 正则表达式引擎，并预设使用。它支持以下语法。
 
 |语法|描述|
 |------|-----------|

include/rapidjson/internal/regex.h

@@ -70,6 +70,10 @@ static const SizeType kRegexInvalidRange = ~SizeType(0);
     - \c \\r Carriage return (U+000D)
     - \c \\t Tab (U+0009)
     - \c \\v Vertical tab (U+000B)
+
+    \note This is a Thompson NFA engine, implemented with reference to 
+        Cox, Russ. "Regular Expression Matching Can Be Simple And Fast (but is slow in Java, Perl, PHP, Python, Ruby,...).", 
+        https://swtch.com/~rsc/regexp/regexp1.html 
 */
 template <typename Encoding, typename Allocator = CrtAllocator>
 class GenericRegex {