need help with Pointer Bounds Checking documentation

[Sandra Loosemore]

The section "Pointer Bounds Checker builtins" in extend.texi is on my 
list for being in need of copy-editing, but reading through the existing 
text, I am quite confused.  In several places it refers to turning the 
Pointer Bounds Checker off, but how do you do that?  I don't see any 
documented command-line option.

It looks like the commit that added this feature (r217125) included a 
lot of x86-specific stuff.  Is this whole feature specific to x86?  If 
so, the documentation ought to say that.  What is the relation of this 
feature to the -mmpx x86 option, which was added in the same commit? 
That option does have documentation in invoke.texi but nothing that 
suggests it's related to Pointer Bounds Checking.

-Sandra the clueless