NF: explicitly introduce "borrowck"

src/borrow-splitting.md

@@ -1,10 +1,10 @@
 # Splitting Borrows
 
 The mutual exclusion property of mutable references can be very limiting when
-working with a composite structure. The borrow checker understands some basic
+working with a composite structure. The borrow checker (a.k.a. borrowck)
-stuff, but will fall over pretty easily. It does understand structs
+understands some basic stuff, but will fall over pretty easily. It does
-sufficiently to know that it's possible to borrow disjoint fields of a struct
+understand structs sufficiently to know that it's possible to borrow disjoint
-simultaneously. So this works today:
+fields of a struct simultaneously. So this works today:
 
 ```rust
 struct Foo {