Add documentation to load_variables() about datasets

[nathanckuhn]

First, thank you to all involved in developing this package!

For load_variables, there is no documentation for what the "dataset" acronyms mean.

I had to learn a lot more about the Census API & dig through this repository's issues before I found an answer. I believe most descriptions of these datasets can be found here: https://api.census.gov/data/

Could some text please be added to the load_variables documentation to provide the link and more context for newer users?

Example: The second paragraph under Details would be more informative if divided into decennial & non-decennial groups and rewritten like the following: (** Please check my work on this! I'm not an expert.)

Datasets available are as follows:

For the Decennial Census: "sf1", "sf2", "sf3", "sf4", "pl", "dhc", "dp", "ddhca", "as", "gu", "mp", "vi", "dpas", "dpgu", "dpmp", "dpvi", "dhcvi", "dhcgu", "dhcvi", "dhcas", "sf2profile", "sf3profile", "sf4profile", "cd110h", "cd110s", "cd110hprofile", "cd110sprofile", "sldh", "slds", "sldhprofile", "sldsprofile", "cqr", "cd113profile", "cd115profile", "cd116", "cd118", and "plnat".

For the American Community Survey: "acsse", "acs1", "acs3", "acs5", "acs1/profile", "acs3/profile", "acs5/profile", "acs1/subject", "acs3/subject", "acs5/subject", "acs1/cprofile", and "acs5/cprofile".

In both the decennial census and ACS, depending on the vintage: "aian", "aianprofile", "cd113", and "cd115".

For the title and description of each, search for the dataset name here: https://api.census.gov/data/
Exception, for ACS dataset names with a forward slash, search for each part separately (e.g., "acs1" and "profile").

** Please note I could not find "dhca" -- it is possible it is no longer provided through the API. If it still is, it would need different documentation.

Thank you again!