Expand gc-stats help

[ju-sh]

Add information about gc-stats to --help command, using information already present in the README.

Made as part of OCaml hack event at Chennai on Oct 21st.

With help from Siva.

[ju-sh]

What more can be added to this? I guess examples from the README needs to be.

We shouldn't be adding them as such, right?

[Sudha247]

Thanks for the PR @ju-sh! You can possibly add the metrics and descriptions mentioned here: https://github.com/tarides/runtime_events_tools#gc-stats

[ju-sh]

Copy-pasted some info from README to gc-stats subcommand's help message.

olly gc-stats --help now shows:

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SYNOPSIS
       olly gc-stats [--json] [--output=output] [OPTION]… EXECUTABLE

DESCRIPTION
       Report the GC latency profile.

       Wall time
           Real execution time of the program

       CPU time
           Total CPU time across all domains

       GC time
           Total time spent by the program performing garbage collection
           (major and minor)

       GC overhead
           Percentage of time taken up by GC against the total execution time

       GC time per domain
           Time spent by every domain performing garbage collection (major and
           minor cycles). Domains are reported with their domain ID (e.g.
           `Domain0`)

       GC latency profile
           Mean, standard deviation and percentile latency profile of GC
           events.

ARGUMENTS
       EXECUTABLE (required)
           Executable (and its arguments) to trace. If the executable takes
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Is it better?

[Sudha247]

Thanks! This looks good to me.