[V8][PATCH 2/2] Update documentation to clarify a GCC extension [PR77650]

[Joseph Myers]

The documentation in this case is OK, though claims about how a future 
version will behave have a poor track record (we tend to end up with such 
claims persisting in the documentation even though the change in question 
didn't get made and might sometimes no longer be considered desirable).

-- 
Joseph S. Myers
joseph@codesourcery.com