testing: document -test.benchmem and ReportAllocs output format

Filing this bug on behalf of someone who prefers not to do it directly:

The ⁠(*testing.B).ReportAllocs function's documentation says that its output is the same as setting -test.benchmem. However, there is as far as I can tell no explanation anywhere of what, exactly, -test.benchmem is or what it does. There is also no explanation as to what the output format is. For instance, in an output line like

BenchmarkF-34         	    9718	    672043 ns/op	   34474 B/op	     508 allocs/op

I can see the lines with four numbers, but one of them is completely dimensionless and the others aren't entirely clear.