What are incompatible upgrades?

I recently came across this nice extension and really would love to use it more.

However, I frequently have to revisit some dependencies I'd like to upgrade due to them claimed to be incompatible. There is even a switch to control whether or not such upgrades are allowed.

What does it mean if something is judged to be incompatible? Most updates worked for me just fine when forced.

Unfortunately, I couldn't find any documentation or explanation on this. Ideally, this would be covered by the help text, but adding some section to `README.md` would be helpful too.

Start with README.md and the existing command help text referenced in the issue, then trace how cargo-edit determines that an upgrade is incompatible and how the override switch is described. Document the meaning of incompatible upgrades and the effect of allowing them, with consistent wording in the help text or README.

Written by the indexing model from the issue text.