tweak docs

Author: RalfJung

src/tools/miri/README.md

@@ -538,7 +538,9 @@ extern "Rust" {
     fn miri_start_panic(payload: *mut u8) -> !;
 
     /// Miri-provided extern function to get the internal unique identifier for the allocation that a pointer
-    /// points to. This is only useful as an input to `miri_print_borrow_stacks`, and it is a separate call because
+    /// points to. If this pointer is invalid (not pointing to an allocation), interpretation will abort.
+    ///
+    /// This is only useful as an input to `miri_print_borrow_stacks`, and it is a separate call because
     /// getting a pointer to an allocation at runtime can change the borrow stacks in the allocation.
     /// This function should be considered unstable. It exists only to support `miri_print_borrow_stacks` and so
     /// inherits all of its instability.
@@ -549,6 +551,7 @@ extern "Rust" {
     /// The format of what this emits is unstable and may change at any time. In particular, users should be
     /// aware that Miri will periodically attempt to garbage collect the contents of all stacks. Callers of
     /// this function may wish to pass `-Zmiri-tag-gc=0` to disable the GC.
+    ///
     /// This function is extremely unstable. At any time the format of its output may change, its signature may
     /// change, or it may be removed entirely.
     fn miri_print_borrow_stacks(alloc_id: u64);