Better document iceberg JWT pass-through

[tokoko]

This is not really an issue with the docs itself, but the closest thing to a documentation for iceberg rest catalog jwt pass-through that we were able to find was a blog post on celerdara.com. The example in the blog shows create catalog command with iceberg.catalog.security set to jwt. However, the actual code in IcebergRESTCatalog that initiates pass-through compares the value to uppercase JWT.

Either making a comparison case-insensitive or better documenting the feature in the docs (and the blog post) would clear up the confusion.