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

Presenting more than one way to satisfy the single-weak-ref requirement leads to
confusing messaging for the end user. Use the introduction of a single unused
weak variable as the preferred solution. This differential modifies D146745.

rdar://103453678

Reviewed By: yln, thetruestblue

Differential Revision: https://reviews.llvm.org/D147526