Revise mention of using the global allocator

Allow for the possibility that the global allocator is the system allocator.

Co-authored-by: Mark Rousskov <mark.simulacrum@gmail.com>

Author: joshtriplett

library/core/src/alloc/global.rs

@@ -125,7 +125,7 @@ use crate::{cmp, ptr};
 ///
 /// Generally speaking for this reason one should stick to library features available through
 /// [`core`], and avoid using [`std`] in a global allocator. A few features from [`std`] are
-/// guaranteed to not use the global allocator:
+/// guaranteed to not use `#[global_allocator]` to allocate:
 ///
 ///  - [`std::thread_local`],
 ///  - [`std::thread::current`],