Document units of memory and time properties of Trial type

Running a benchmark produces an instance of the Trial Type.

When running on the REPL it is nice that you can see a pretty-printed version of the results, however it is not as straightforward to save those results to a text file.

The manual states that we can calculate the metrics ourselves by using the minimum,median,mean,maximum, and std functions, i.e.

julia> maximum(t)
BenchmarkTools.TrialEstimate: 
  time:             1.503 ms
  gctime:           1.401 ms (93.21%)
  memory:           16.36 KiB
  allocs:           19

What it is not immediately obvious is the actual unit that the metrics are saved as internally. Through trial an error I found out that memory is stored as bytes and time is stored as nanoseconds, i.e.

julia> mean(t).time
1.503e6

Start with the linked manual section on handling benchmark results and the Trial and TrialEstimate properties described in the issue. Document the internal units for memory and time, including the byte and nanosecond values, so users can interpret metrics returned by functions such as mean and maximum.