In Work Items, Description and Comments, is there a way to specify
monospace text (for code or ASCII tables)?

There is a request tracking this issue:
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/94186

Looks like in the web interface, HTML tags such as pre, code, b, and u
are recognized and unrecognized tags are filtered out.

In the Web UI, only plain text is supported at the moment.
Rich text editing is tracked in:

https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/94072
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/63151

In the eclipse client, I see how to bold and italicize in the Edit
menu, but the same html tags recognized in the Web interface seem to
have no effect (and html tags are rendered as-is, without any special
recognition).

I don't think the tags are supported in the Web UI, as we only support
plain text there. Maybe the preview mistakenly interprets them, but when
saving the work item, all tags you enter will be escaped.

Is there documentation about how text can be formatted in each
interface? I've been searching but unable to find any.

The Eclipse UI supports bold and italic, the Web UI only plain text at
the moment.

(It's not without some irony that I note how this forum's edit
capabilities in terms of HTML and BBCode, etc are much more
straight-forward, with links to documentation about BBCode.)

Note that there are also plans to support Wiki syntax in work item
fields, e.g.:
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/87341

--
Regards,
Patrick
Jazz Work Item Team