Are you a member of confluence-writers? This should be closed to comments as the docs have moved to gitbook.
In any case altering a comment is beyond the scope of the 9.1 and earlier alter support - but should be available with TEIID-2578.
On a create you use the ANNOTATION option to set the description, which is shown on the DDL Metadata page.
I see no method for COMMENTS (DESCRIPTIONS). There should be a method to include comments/descriptions on all objects both at CREATE as well as ALTER.