Make a note about "gigabyte"

Fixes rust-lang#28461

src/doc/trpl/the-stack-and-the-heap.md

@@ -74,7 +74,9 @@ visualize what’s going on with memory. Your operating system presents a view o
 memory to your program that’s pretty simple: a huge list of addresses, from 0
 to a large number, representing how much RAM your computer has. For example, if
 you have a gigabyte of RAM, your addresses go from `0` to `1,073,741,823`. That
-number comes from 2<sup>30</sup>, the number of bytes in a gigabyte.
+number comes from 2<sup>30</sup>, the number of bytes in a gigabyte. [^gigabyte]
+
+[^gigabyte]: ‘Gigabyte’ can mean two things: 10^9, or 2^30. The SI standard resolved this by stating that ‘gigabyte’ is 10^9, and ‘gibibyte’ is 2^30. However, very few people use this terminology, and rely on context to differentiate. We follow in that tradition here.
 
 This memory is kind of like a giant array: addresses start at zero and go
 up to the final number. So here’s a diagram of our first stack frame: