Fix StsProperties#asyncCredentialsUpdate documentation

[MatejNedic]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change
• All tests passing
• No breaking changes

🔮 Next steps

[maciejwalkowiak]

When we are here, perhaps StsProperties#isAsyncCredentialsUpdate field should be renamed to asyncCredentialsUpdate and i guess this will make javadoc properly appear in configprops asciidoc.