External References: Explain Comments and Hashes

[msymons]

In 0x51-External-References.md...

• Add coverage of the comment field. Ideally, there should also be a use-case that can demonstrate how the comment might deliver real value.
• Add coverage of hashes over and above the example already provided. The specification says that hashes are used "if applicable". Thus, it would be useful to explain in the guide when hashes are applicable... and when they are not.

It would also be ueful if the table had an additional column to detail in which version of the specification each External Reference first appeared. eg, I just wasted an hour or two trying to work out why static-analysis-report was giving problems... having not realized that my generating tool only supports up to v1.4 format and the problem reference was introduced in v1.5.