testing: improve the text of the b.Loop documentation #70787

// Loop returns true until b.N calls has been made to it.
//
// A benchmark should either use Loop or contain an explicit loop from 0 to b.N, but not both.
// After the benchmark finishes, b.N will contain the total number of calls to op, so the benchmark
// may use b.N to compute other average metrics.
//
// The parameters and results of function calls inside the body of "for b.Loop() {...}" are guaranteed
// not to be optimized away.
// Also, the local loop scaling for b.Loop ensures the benchmark function containing the loop will only
// be executed once, i.e. for such construct:
//
//	testing.Benchmark(func(b *testing.B) {
//			...(setup)
//			for b.Loop() {
//				...(benchmark logic)
//			}
//			...(clean-up)
//	}
//
// The ...(setup) and ...(clean-up) logic will only be executed once.
// Also benchtime=Nx (N>1) will result in exactly N executions instead of N+1 for b.N style loops.
func (b *B) Loop() bool