rust: Add comment inside block [PR119342]

rust: Add comment inside block [PR119342]

Inside a BLOCK node, all of the variables of the scope/block
are chained together and that connects them to the block.
This just adds a comment to that effect as reading the code
it is not so obvious why they need to be chained together.

gcc/rust/ChangeLog:

	PR rust/119342
	* rust-gcc.cc (block): Add comment on why chaining
	the variables of the scope toether.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>