doc: update comments of code.

- add lost testbench for wchar encoding convertion.
- update code documentation.

src/EncodingHelper.hpp

@@ -9,6 +9,45 @@
 #include "WinImportSuffix.hpp"
 #endif
 
+/**
+ * @brief The namespace handling encoding issues.
+ * @details
+ * \par Windows Encoding Convertion
+ * This namespace provides the convertion between wchar_t, UTF8 and code-page-based string:
+ * The function name has following format: \c AAAToBBB.
+ * AAA is the source string and BBB is target string.
+ * AAA and BBB has following possible value:
+ * \li \c Char: Code-page-based string. Usually it will add a code page parameter for function to get the code page of this string. For code page, please see Microsoft document.
+ * \li \c UTF8: UTF8 string.
+ * \li \c Wchar: wchar_t string.
+ * \par
+ * For example: \c WcharToUTF8 will perform the convertion from wchar_t to UTF8, 
+ * and \c CharToChar will perform the convertion between 2 code-page-based string and caller can specify individual code page for these 2 string.
+ * \par
+ * These functions are Windows specific and are unavailable on other platforms.
+ * Becasue Windows use wchar_t string as its function arguments for globalization, and this library use UTF8 everywhere.
+ * So it should have a bidirectional way to do convertion between wchar_t string and UTF8 string.
+ * 
+ * \par UTF32, UTF16 and UTF8 Convertion
+ * This namespace also provide the convertion among UTF32, UTF16 and UTF8.
+ * These convertion functions are suit for all platforms, not Windows oriented.
+ * \par
+ * Due to implementation, this library assume all non-Windows system use UTF8 as their C locale.
+ * Otherwise these functions will produce wrong result.
+ * 
+ * \par Function Parameters
+ * We provide these encoding convertion functions with following 2 types:
+ * \li Function returns \c bool and its parameter order source string pointer and a corresponding \c std::basic_string container for receiving result.
+ * \li Function returns corresponding \c std::basic_string result, and its parameter only order source string pointer.
+ * \par
+ * For these 2 declarations, both of them will not throw any exception and do not accept nullptr as source string.
+ * The only difference is that the way to indicate convertion error.
+ * \par
+ * First declaration will return false to indicate there is an error when doing convertion. Please note that the content of string container passing in may still be changed!
+ * Last declaration will return empty string to indicate error. Please note if you pass empty string in, they still will output empty string but it doesn't mean an error.
+ * So last declaration is used in the scenario that we don't care whether the convertion success did. For example, output something to console.
+ * 
+*/
 namespace YYCC::EncodingHelper {
 
 #if YYCC_OS == YYCC_OS_WINDOWS