Documentation JARs of Grails installed with SDKMAN! not detected

### What steps will reproduce the issue?
1. Install Grails using SDKMAN!
2. Configure a Grails project in IDEA to use this SDK:
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#### What is the expected result?

Classes, sources, and documentation are found and included ein the project dependencies.

#### What happens instead?

Documentation is not picked up.
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Note that the JavaDocs _are_ there:
```
$ ls -1 .sdkman/candidates/grails/2.5.6/dist/*-javadoc.jar | wc -l
44
```
#### Workaround

Enter manually into Paths > JavaDoc:
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*3 attachment(s) referenced here (`image.png`, `image1.png`, `image3.png`) are not part of the CSV export.*
*Imported from [IDEA-200707](https://youtrack.jetbrains.com/issue/IDEA-200707) · Type: Bug · Votes: 0*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start by reproducing the issue with a Grails 2.5.6 SDK installed through SDKMAN! and inspect the plugin's SDK and JavaDoc discovery path. Confirm that the existing documentation JARs are detected and included in the IntelliJ IDEA project without manually entering a path.

Written by the indexing model from the issue text.