[analyzer] Extend the documentation of MallocOverflow

Previously by following the documentation it was not immediately clear
what the capabilities of this checker are.

In this patch, I add some clarification on when does the checker issue a
report and what it's limitations are.
I'm also advertising suppressing such reports by adding an assertion, as
demonstrated by the test3().
I'm highlighting that this checker might produce an extensive amount of
findings, but it might be still useful for code audits.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D107756