Update 02_historical_summary.md

[kbogan]

#59

[regineheberlein]

Based on discussion during the TS-DACS call, there was a suggestion to clarify language surrounding the "verifiability" of facts.

Perhaps we could use "based on documented information" instead?

I really liked Maristella's example of the "father of the electric guitar".

[dacscommunity]

In favor

[milesnimer]

The proposed changes to this element are good practice, but given the current structure of the element's guidelines a revision to DACS 2.7 might be sufficient. Alternatively, it could be helpful to have more structured rules in 11.2 that deal with general biographical/historical notes (e.g., source selection, use of sub-elements) even if they duplicate the content in Part I.

[michelsd]

I agree with @milesnimer that this is probably sufficiently covered by #84.

[weatherlys]

It would be helpful to see the example updated as well, or to provide an additional example that follows the added guidance.