[docs] Update Scudo documentation

Summary:
The documentation has fallen a bit behind, update it with the latest evolution
of the allocator:
- clarify a couple of expectations regarding what is meant to be achieved;
- update the header format;
- document `-fsanitize=scudo`.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41670

llvm-svn: 321811