hashmap: clarify that hashmap_entry can safely be discarded

The API documentation said that the hashmap_entry structure to be
embedded in the caller's structure is to be treated as opaque, which
left the reader wondering if it can safely be discarded when it no
longer is necessary.  If the hashmap_entry structure had references
to external resources such as allocated memory or an open file
descriptor, merely free(3)ing the containing structure (when the
caller's structure is on the heap) or letting it go out of scope
(when it is on the stack) would end up leaking the external
resource.

Document that there is no need for hashmap_entry_clear() that
corresponds to hashmap_entry_init() to give the API users a little
bit of peace of mind.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-hashmap.txt

@@ -104,6 +104,11 @@ If `free_entries` is true, each hashmap_entry in the map is freed as well
 `entry` points to the entry to initialize.
 +
 `hash` is the hash code of the entry.
++
+The hashmap_entry structure does not hold references to external resources,
+and it is safe to just discard it once you are done with it (i.e. if
+your structure was allocated with xmalloc(), you can just free(3) it,
+and if it is on stack, you can just let it go out of scope).
 
 `void *hashmap_get(const struct hashmap *map, const void *key, const void *keydata)`::