You do give a specific purpose, but I'm still struggling to see how what you're asking for would really help. Usually the only consumers of the objects defined by DDL (tables, views, etc.) are going to be other developers working within the same database; quite often, the external interface to the database is better defined by stored procedures, or a similar construct in a database-client layer. While the same issue of documentation remains, it seems to me that the scope is different.

In the past I have seen a table used to store the description of each table/column then a report generator to create the document. A simple unit test can check there is a description for each table/column.

https://stackoverflow.com/questions/5246575/what-is-a-good-way-to-document-what-your-tables-and-columns-mean-in-a-sql-server covers some options for SqlServer

@Canina this is to support examples in published documentation about a SQL platform. I'll edit.

The question is if tool recommendation questions are on-topic or off-topic to begin with. This draft proposal, which is arguably not community consensus (yet) would have made this question off-topic, for the reason "Off-topic: Recommendation questions about which tools, libraries or technologies to use or where to find them."

@Lundin good question, which I hadn't considered. Questions about tools (like javadoc, git, etc) were on-topic on SO, so I made a logical leap here which might not be warranted. At the core I'm trying to solve a technical problem and don't know if a tool exists at all. That feels a little different from "which should I use, git or svn?", for example, but maybe it's still on the wrong side of the line?

Pretty sure it would be closed on SO, they have very low tolerance for "which tools exist". Though if that phrase is simply changed to "how to document SQL DDL" then it should be under the (supposedly) broader scope of this site, which includes software documentation questions and also perhaps "what's best" questions with a well-defined "best".

@Lundin I made some edits based on your comments. Is that better?

Yep, looks good as far as I'm concerned :) If tool recommendations are to be off-topic, we should probably come up with some examples of on-topic/off-topic and place such in the community help.

I'm actually quite interested in this question for myself. We're using Doxygen for the rest of our code, but we don't have any tooling for the DB beyond a schemacrawler script that generates a PDF schema diagram. Notes about how a table is intended to be used are written in un-formatted comments, but that requires reading the code.