Fix several doxygen warnings #1439

Job	Run time
🔨 MSVC x64 cpp11	2m 37s
🔨 MSVC x86 cpp11	2m 58s
🔨 MSVC x64 glib	2m 24s
🔧 MSVC Additional (debug_prof)	2m 23s
🔨 MSVC x86 glib	2m 58s
🔧 MinGW fluidsynth-mingw x86	2m 38s
🔧 MinGW fluidsynth-mingw1 x86	1m 49s
🔧 MinGW fluidsynth-mingw x64	1m 49s
🔧 MSVC Additional (default)	1m 57s
🔧 MinGW fluidsynth-mingw-ffloat-store x86	1m 58s
🔧 MinGW fluidsynth-mingw-static x64	1m 52s
🔧 MSVC Additional (no_network)	2m 26s
🔧 MSVC Additional (minimal)	2m 9s
🔧 MinGW fluidsynth-mingw2 x86	1m 50s
🔧 MSVC Additional (osal_cpp11)	2m 21s
🚧⬛ mingw32 glib	3m 47s
🚧🟨 ucrt64 cpp11	4m 9s
🚧⬛ mingw32 cpp11	3m 47s
🚧🟧 clang64 glib	6m 7s
🚧🟦 mingw64 cpp11	3m 40s
🚧🟦 mingw64 glib	4m 10s
🚧🟨 ucrt64 glib	4m 30s
🔧 MSVC Additional (static_lib)	2m 19s
🚧🟧 clang64 cpp11	7m 11s
🧪 Acceptance Test	5s
🚀 Publish Release Artifacts	0s
📢 Announce Release	0s
	1h 13m 54s

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