Automatic merge from submit-queue
documentation string for DefaultExternalAddress
**What this PR does / why we need it**:
docs for a public function
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
I encountered this function that did not have a doc string. The function was easy to read, so I wrote a doc string for the function.
e7537a21d9