Improve documentation of -UTF8String #2876

Job	Run time
Windows arm64 MSVC Clang gnustep-2.2	36m 36s
Windows x64 MinGW GCC	26m 36s
Windows x64 MSVC Clang gnustep-2.2	1m 15s
Windows x64 MinGW Clang gnustep-2.2	26m 6s
Ubuntu x64 Clang gnustep-2.2	5m 13s
Ubuntu x64 GCC	5m 22s
Rocky Linux 9 x64 Clang gnustep-2.2	6m 28s
Rocky Linux 8 x64 Clang gnustep-2.2	2m 50s
	1h 50m 26s

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