disable-language-runtime-unwindplans desc rewrite to be searchable

The description for disable-language-runtime-unwindplans did not
include likely search terms ("backtrace", "stack"), rewrite it
to include those so it is more easily discoverable with apropos.
The text is still not the clearest description of what a language
runtime is / what it might do, but this is better.

Differential Revision: https://reviews.llvm.org/D142663