Libstdc++: Update documentation about default -std option.
The libstdc++ documentation now correctly describes the default value for the ``-std`` compiler option.
The libstdc++ documentation has been updated to accurately describe the default value for the -std compiler option. This option controls the C++ standard used by the compiler. The update ensures that users understand how the compiler interprets their code by default.
In Details
This commit modifies doc/xml/manual/using.xml (specifically the Flags section) and regenerates doc/html/manual/using.html to update the description of the default value for the -std compiler option.
For Context
This commit updates the libstdc++ documentation to clarify the default value of the -std compiler flag. The -std flag tells the compiler which version of the C++ standard to use when compiling code (e.g., C++11, C++17, C++20). This change ensures the documentation accurately reflects the compiler's default behavior if no -std flag is provided.