[docs] Scudo documentation minor update

Summary:
Minor changes to reflect changes to the code that were not documented:
- `SCUDO_DEFAULT_OPTIONS` compile time way of defining options;
- MIPS added as a supported architecture;
- clarification on how to fully disable the Quarantine;
- rewording in a few places.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: sdardis, arichardson, atanasyan, llvm-commits

Differential Revision: https://reviews.llvm.org/D47071

llvm-svn: 332736