doc: clarify expected memory management strategy

Right now, docs don't make it clear when exactly does it become okay to
free memory belonging to `uv_handle_t`. It's only stated that
`uv_close` must be called before freeing the memory, which is a source
of confusion for new users: they call `uv_close(handle, NULL)`, then
free the memory (see e.g. #2078, https://stackoverflow.com/q/25615340).

PR-URL: https://github.com/libuv/libuv/pull/2087
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

docs/src/handle.rst

@@ -140,6 +140,8 @@ API
 
     Request handle to be closed. `close_cb` will be called asynchronously after
     this call. This MUST be called on each handle before memory is released.
+    Moreover, the memory can only be released in `close_cb` or after it has
+    returned.
 
     Handles that wrap file descriptors are closed immediately but
     `close_cb` will still be deferred to the next iteration of the event loop.