Improve documentation about setting performance reference values

[vkarak]

From Slack discussion here:

At https://reframe-hpc.readthedocs.io/en/stable/tutorial_basics.html#adding-reference-values - I always struggle to understand what the performance value tuple will result in and always resort to making a test where it fails to understand how the tuple works. I'd find it helpful to have a few examples containing the tuple and what this results as in lower and upper value. I was thinking something like:

Performance Tuple               Expected      Lowest accepted   Highest accepted
(25200, -0.05, 0.05, 'MB/s')    25200 MB/s    23940 MB/s        26460 MB/s
(40, None, 0.1, 'GB/s')         40 GB/s       0                 44 GB/s
(100, -0.1, None, 'Gflop/s')    100 Gflop/s   99 Gflop/s        inf