GPUUploadManager: improve docs and pending update size tracking #4143

Job	Run time
Linux -> Pre-checks	16s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 56s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 2s
Linux -> GCC-x64, Debug	11m 57s
Linux -> Clang-x64, Debug	13m 3s
Linux -> Clang-x64, Release	12m 53s
Linux -> GCC-x64, Release	10m 28s
	1h 3m 35s

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