[ASan] Minor documentation fix: clarify static linking limitation.

Summary:
ASan does not support statically linked binaries, but ASan runtime itself can
be statically linked into a target binary executable.

Reviewers: eugenis, kcc

Reviewed By: eugenis

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D55066

llvm-svn: 348863