SDL_filesystem.h: Fill in missing documentation. #3059

Job	Run time
Create test plan	4s
level1 / Haiku	1m 51s
level2 / GDK (MSVC, x64)	2m 28s
level2 / MacOS (Framework) (arm64)	1m 2s
level2 / MacOS (GNU prefix)	1m 19s
level2 / Ubuntu 20.04 (Intel Compiler)	4m 39s
level2 / Windows (msys2, clang32)	2m 17s
level2 / MacOS (Framework) (x64)	2m 3s
level2 / Android (Gradle)	2m 17s
level2 / Sony PlayStation Vita (GLES w/ pib)	3m 13s
level2 / Windows (msys2, mingw64)	8m 2s
level2 / Ubuntu 22.04	4m 36s
level2 / Nintendo 3DS	2m 3s
level2 / Android (Android.mk)	1m 19s
level2 / Windows (MSVC, x64)	4m 16s
level2 / Sony PlayStation 2	5m 3s
level2 / iOS (CMake & xcode)	53s
level2 / Windows (msys2, clang64)	2m 7s
level2 / Steam Linux Runtime (Sniper)	3m 2s
level2 / Sony PlayStation Vita (GLES w/ PVR_PSP2)	3m 23s
level2 / Emscripten	2m 21s
level2 / PowerPC	1m 54s
level2 / NetBSD	2m 35s
level2 / Windows (msys2, mingw32)	8m 29s
level2 / tvOS (CMake & xcode)	55s
level2 / Sony PlayStation Portable	3m 54s
level2 / Windows (MSVC, clang-cl x64)	3m 2s
level2 / Ubuntu 20.04 (Intel oneAPI)	3m 3s
level2 / FreeBSD	1m 54s
level2 / Windows (MSVC, ARM)	2m 56s
level2 / PowerPC64	1m 48s
level2 / Windows (MSVC, x86)	5m 38s
level2 / Windows (MSVC, clang-cl x86)	2m 32s
level2 / Ubuntu 20.04	4m 6s
level2 / Windows (msys2, ucrt64)	8m 18s
level2 / Windows (MSVC, ARM64)	3m 45s
level2 / LoongArch64	2m 45s
level2 / Android (CMake)	50s
level2 / RISC OS	1m 25s
level2 / Android (CMake, lean)	1m 6s
	1h 59m 13s

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