METRON-643: Stellar function documentation needs to be updated

[JonZeolla]

The documentation for some the STATS_* Stellar functions was moved from here to here as of this commit but the documentation was not correctly updated.

This PR updates the core function table in metron-common to point to the new locations, and fixes some minor typos.

The alternative would be to simply remove the STATS_* entries from the table in metron-common, and to add a pointer to the other locations where Stellar functions are documented. Open to feedback.

[JonZeolla]

Note: BIN and STATS_BIN may be added via METRON-637 but the documentation in metron-common was not updated, potentially indicating a preference to remove the STATS_* documentation from the metron-common README.md.

I would suggest that at the end of the day we make sure there's a single place where people can go for Stellar function documentation, even if it's just a page of pointers to the various READMEs.

[nickwallen]

+1 Looks good, Jon.

I agree with you that we need one single, searchable "thing" documenting all of the Stellar functions. The approach that you took here gives us that. I prefer your approach over simply having one link to the 'metron-statistics' README which would make a user click through to multiple READMEs and search each one.

Longer term, we really need something along the lines of auto-generated documentation that gets published with each release. The approach we currently have is difficult to maintain and is destined to fall out-of-sync from the underlying code.

[cestella]

@JonZeolla I agree; I like this approach. I will commit and update the STATS_BIN PR to conform to the new approach.