[sanitizers] Explainer about dyld and weak overrides on Darwin. (NFC)

Explain in the release notes that the Darwin dynamic linker (dyld) requires
that at least one weak symbol be present in any mach-o file that defines an
intended override of a sanitizer dylib weak reference.

rdar://103453678

Reviewed By: thetruestblue

Differential Revision: https://reviews.llvm.org/D146745