Specify the meaning signal 6 in vector documentation

[Chaldron]

A lot of students fail their own asserts and ultimately end up seeing a crash relating to signal 6 in the AG output. We should specify in vector's documentation that if they see a signal 6, their program was killed because they failed their own assert. We don't need it in every documentation, but specifically for vector since that's when the students are first instructed to use asserts.