[docs] Documentation update for Scudo

Summary:
Documentation update to reflect the changes that occured in the allocator:
- additional architectures support;
- modification of the header;
- options default values for 32 & 64-bit.

Reviewers: kcc, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29592

llvm-svn: 294595