[FR] Improve docs for naming of benchmark arguments

[pdroalves]

Is your feature request related to a problem? Please describe.
I have benchmarks that need to be executed for different input parameter sets composed by many integer attributes. Today the output line for each benchmark is just the concatenation of the fixture name, the benchmark name, and all the numerical attributes. For instance:

MyFixture/MyImportantBenchmark/744/2048

This is terrible to read since the user may not know what each number relates to.

Describe the solution you'd like
I would like to have the possibility to use a name generator, just as googletest has, to customize this. So that line could become

MyFixture/MyImportantBenchmark/foo_744/bar_2048

[LebedevRI]

That already works, see ArgName() and ArgNames() (#305)
Please consider contributing a documentation for this, since it is clearly missing.