doc: GTY((cache)) documentation tweak

gcc/ChangeLog:

	* doc/gty.texi: Add discussion of cache vs. deletable.

gcc/doc/gty.texi

@@ -306,6 +306,13 @@ called on that variable between the mark and sweep phases of garbage
 collection.  The gt_clear_cache function is free to mark blocks as used, or to
 clear pointers in the variable.
 
+In a hash table, the @samp{gt_cleare_cache} function discards entries
+if the key is not marked, or marks the value if the key is marked.
+
+Note that caches should generally use @code{deletable} instead;
+@code{cache} is only preferable if the value is impractical to
+recompute from the key when needed.
+
 @findex deletable
 @item deletable