[Improvement] User can create a paimon schema with comment ,but get the schema missing "comment" field

### What would you like to be improved?

**1. Create a paimon catalog use jdbc as backend**

**2. create a paimon schema with comment**

**3. get the schema in step 2, missing comment**

**4. The doc only specify "Doesn't support store schema comment for FilesystemCatalog."**

### How should we improve?

Align the document with code.

Currently ,the doc:
Doesn't support store schema comment for FilesystemCatalog.

Actually , the paimon catalog doesn't support store schema comment, no matter what's the backend

Start by locating the documentation text describing schema comments for FilesystemCatalog and compare it with the reported behavior for Paimon catalogs using JDBC and other backends. Update the documentation so its scope matches the code, then review the rendered wording for consistency.

Written by the indexing model from the issue text.