docs: clarify pointer lifetimes for ktxTexture_SetImageFromMemory #818

Job	Run time
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Debug,Release, ON, ON, OpenGL+Vulk...	24m 22s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	4m 18s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	5m 16s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	5m 15s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Debug,Release, ON, ON, OpenGL+V...	28m 17s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	7m 18s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	6m 30s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	6m 50s
windows (windows-2019, Visual Studio 16 2019, v142, x64, Debug,Release, ON, ON, OpenGL+Vulkan, ON...	25m 31s
windows (windows-latest, Visual Studio 17 2022, CLangCL, arm64, Debug,Release, ON, ON, OpenGL, OF...	29m 52s
	2h 23m 29s

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