zephyr: alloc: Add some documentation to the allocator

Write docs for the global allocator.  This will also trigger bringing in
the documentation for the module, which explains how to use the
allocator.

Signed-off-by: David Brown <[email protected]>

zephyr/src/alloc_impl.rs

@@ -62,6 +62,10 @@ extern "C" {
     fn free(ptr: *mut u8);
 }
 
+/// An allocator that uses Zephyr's allocation primitives.
+///
+/// This is exported for documentation purposes, this module does contain an instance of the
+/// allocator as well.
 pub struct ZephyrAllocator;
 
 unsafe impl GlobalAlloc for ZephyrAllocator {
@@ -83,5 +87,6 @@ unsafe impl GlobalAlloc for ZephyrAllocator {
     }
 }
 
+/// The global allocator built around the Zephyr malloc/free.
 #[global_allocator]
-static ZEPHYR_ALLOCATOR: ZephyrAllocator = ZephyrAllocator;
+pub static ZEPHYR_ALLOCATOR: ZephyrAllocator = ZephyrAllocator;