[MODORDERS-966] - Add description to acq unit dto schema

[imerabishvili]

Purpose

https://issues.folio.org/browse/MODORDERS-966

Approach

Add the corresponding field.

Pre-Merge Checklist

Before merging this PR, please go through the following list and take appropriate actions.

• Does this PR meet or exceed the expected quality standards?
  • Examples exist for all schemas
  • Descriptions exist for all schema properties
  • All schemas pass raml-cop linting
• Does this introduce breaking changes?
  • Were there any schema changes?
  • There are no breaking changes in this PR.

If there are breaking changes, please STOP and consider the following:

• What other modules will these changes impact?
• Do JIRAs exist to update the impacted modules?
  • If not, please create them
  • Do they contain the appropriate level of detail? Which endpoints/schemas changed, etc.
  • Do they have all they appropriate links to blocked/related issues?
• Are the JIRAs under active development?
  • If not, contact the project's PO and make sure they're aware of the urgency.
• Do PRs exist for these changes?
  • If so, have they been approved?

Ideally all of the PRs involved in breaking changes would be merged in the same day to avoid breaking the folio-testing environment. Communication is paramount if that is to be achieved, especially as the number of intermodule and inter-team dependencies increase.

While it's helpful for reviewers to help identify potential problems, ensuring that it's safe to merge is ultimately the responsibility of the PR assignee.