[ThinLTO] Correct documentation on default number of threads

Summary:
The number of threads used for ThinLTO backend parallelism was
dropped to the number of cores in r284618 to avoid oversubscribing
physical cores due to hyperthreading. This updates the documentation
to reflect that change.

Fixes PR38610.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50882

llvm-svn: 340021