Document the valid values of the "supports" column for each data feed

[leeevans]

Create a table in the LAERTES documentation to show the valid values of the "supports" column for each data feed.

[ericaVoss]

I see t, f, and NULL.

The MEDLINE MeSH EVIDENCE_TYPES are now NULL.

@rkboyce is there a reason MEDLINE MeSH is NULL? I find NULLs can be dangerous depending on how the DB deals with them, might want to add the t back in.

[rkboyce]

NULL means that there is no way to tell if the evidence supports or refutes. We could use a character instead of NULL such as 'u'. Lets do that in another revision.