--benchmark_min_time command line option undocumented

[kpet]

The documentation mentions it once but I couldn't find comprehensive documentation for it. As far as I can tell, this option can be used for two things

1. Set the min duration of a repetition with --benchmark_min_time=<value> or --benchmark_min_time=<value>s.
2. Set the desired number of iterations per repetition with --benchmark_min_time=<value>x (for benchmarks not registered with a fixed number of iterations).

This seems like it relates to #746

Are both of these bits of behaviour officially supported? If yes, it would be helpful to document them.

[dmah42]

if they're in the code, they're supported. if we're missing some documentation, i'm happy to take PRs improving the docs.