An architecture description should include the design rationale behind the design decisions being documented. In Chapter 5, Designing Software Architectures, we discussed the design rationale, which is an explanation that contains the reasons and justification for design decisions related to the architecture. Without documenting the design rationale, the reasons that a design decision was made will not be known. Recording the design rationale is beneficial even for those who are involved in a design decision, as the details of a decision can be forgotten over time.
It is not necessary (or practical) to record every design decision that is made but any decisions that are important to the architecture are candidates to be documented. When documenting a design rationale, keep in mind that in addition to including design decisions, it is sometimes useful to include details on why alternative approaches were not taken and why certain design decisions were not made at all.