Apparently ? is an option in some contexts, such as key generation. I cannot
find any mention of this in the documentation, at least not easily.
The users that need "?" the most, are also users that typically won't read
through all of the documentation. So, the ability to use "?" should be noted
somewhere more obvious.
For example, this could be added to the man page section for --gen-key and
addkey which is currently quite sparse. Alternatively, this could be mentioned
at the start of key generation.