Revisit how users can find a learn to use the `eml$*` constructors

[amoeba]

[I didn't find a ticket related to this but let me know if there is one]

You can create EML with this package using three modalities and those modalities can be mixed together:

1. plain old lists
2. set_* methods
3. eml$* constructors

They all require you to know a decent amount of the EML standard to use and probably look things up as you work. However, sometimes I think you know what element you need but not what child elements it takes. This is where the eml$* constructors are useful.

That said, they're not well-documented. ?eml gives you a nearly blank help page and I'm not sure there's help elsewhere.

Could we make this situation better? I had a few ideas:

• Look at where the README, any R help pages, and any vignettes stand and introduce the eml$* constructors early and make it easy to find if you forget
• Create in a ?EML help page
• Create a help page for the eml$* constructor
• Create help pages for EML elements, like ?coverage, ?creator (possibly namespaced with eml_ like ?eml_creator)

Please add in any ideas or ask questions.

[mbjones]

The discussion of auto-generated docs for eml$ is in https://github.com/ropensci/EML/issues/252#issuecomment-442595315

[cboettig]

yup, with prototype over at https://cboettig.github.io/build.eml/reference/eml.html, but still needs more test-driving to figure out what issues arise from this approach. (Some we've discussed in https://github.com/cboettig/build.eml/issues/2)