Misc fixes: server bind address, Sequence[] documentation

This is a rollup of two small patches:

- Test (and document) how `Sequence` behaves if it's inside a held argument
- Fix mathicsserver binding to the wildcard address even if `-e` isn't given

The bind address could be considered a security issue because it meant that in effect, `-e` was always enabled by default. I didn't add anything to the changelog, but it might be appropriate if it's worth releasing another RC for this. Let me know and I'll push a patch to do that.