Clarify when cargo detects changes

Author: TheButlah

src/doc/src/reference/build-scripts.md

@@ -323,7 +323,7 @@ cross-compiling, so keep that in consideration of the impact on compile time.
 
 When rebuilding a package, Cargo does not necessarily know if the build script
 needs to be run again. By default, it takes a conservative approach of always
-re-running the build script if any file within the package is changed (or the
+re-running the build script if any rust file within the package is changed (or the
 list of files controlled by the [`exclude` and `include` fields]). For most
 cases, this is not a good choice, so it is recommended that every build script
 emit at least one of the `rerun-if` instructions (described below). If these