DISASSEMBLE-MEMORY should not fail an AVER about unboxed constants.

Unless disassembling either a code object or function, the disassembler
typically blows past the 'opcodes-length' slot of a disassem segment.
So just return without complaint.
Fix structure documentation.

Incorrect usage of #-sb-xc-host prevented structure docstrings from
being attached to SBCL structures.