guarantees: Clarify purpose with respect to performance security and tender security

[duncandewhurst]

Based on the descriptions in open-contracting/standard#651 (comment), my understanding is that:

• Security for the performance of the procurement contract should be modelled using the guarantees extension
• Tender (bid) security should be modelled using the participation fees extension with .type set to 'deposit'

I think it would be helpful to add some clarification to the documentation of the guarantees extension to signpost implementers to the participation fees extension for bid security.

[jpmckinney]

Great idea! Let's do that.

[jpmckinney]

The PR has been opened on the third-party repository, but it's kind of out of our hands. Closing here.