docs: fix URL in c_gpu, and opts explain upsampfac choices better #392

Job	Run time
prepare	3s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=native)	1m 9s
Build with Pip (ubuntu-22.04, 3.8, llvm, -march=x86-64)	1m 5s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=native)	46s
Build with Pip (ubuntu-22.04, 3.8, gcc, -march=x86-64)	46s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=native)	1m 10s
Build with Pip (ubuntu-22.04, 3.11, llvm, -march=x86-64)	1m 18s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=native)	46s
Build with Pip (ubuntu-22.04, 3.11, gcc, -march=x86-64)	46s
Build with Pip (windows-2022, 3.8, msvc, /arch:AVX2)	11m 53s
Build with Pip (windows-2022, 3.8, msvc, /arch:SSE2)	11m 52s
Build with Pip (windows-2022, 3.8, llvm, /arch:AVX2)	11m 25s
Build with Pip (windows-2022, 3.8, llvm, /arch:SSE2)	12m 36s
Build with Pip (windows-2022, 3.11, msvc, /arch:AVX2)	12m 5s
Build with Pip (windows-2022, 3.11, msvc, /arch:SSE2)	11m 42s
Build with Pip (windows-2022, 3.11, llvm, /arch:AVX2)	12m 39s
Build with Pip (windows-2022, 3.11, llvm, /arch:SSE2)	12m 15s
Build with Pip (macos-13, 3.8, llvm, -march=native)	2m 51s
Build with Pip (macos-13, 3.8, llvm, -march=x86-64)	3m 56s
Build with Pip (macos-13, 3.8, gcc-14, -march=native)	1m 24s
Build with Pip (macos-13, 3.8, gcc-14, -march=x86-64)	1m 43s
Build with Pip (macos-13, 3.11, llvm, -march=native)	2m 33s
Build with Pip (macos-13, 3.11, llvm, -march=x86-64)	3m 43s
Build with Pip (macos-13, 3.11, gcc-14, -march=native)	1m 59s
Build with Pip (macos-13, 3.11, gcc-14, -march=x86-64)	1m 25s
	2h 3m 50s

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