doc: replace return with retval

`@return` is dedicated for brief description of return values, not for comments
stating actual return values. In addition, sphinx + breathe does not join
multiple adjacent `@return`. This results in multiple `Return` sections in the
generated document, which is confusing.

This patch replaces `@return` with `@retval` for the lists of return
values. Adjacent `@retval` can be joined into one list by breathe.

v1 -> v2:

* Replace return value descriptions like `negative` and `positive` with
  expressions like `<0` and `>0` in `@retval`.
* Keep the list of `@retval` comprehensive, even when there is a `@return` to
  generally describe what the return value means.
* Drop duplicated `@return` when it does not give more information than the
  `@retval` list.

Tracked-On: #1595
Signed-off-by: Junjie Mao <junjie.mao@intel.com>

hypervisor/boot/sbl/multiboot.c

@@ -158,8 +158,8 @@ static void *get_kernel_load_addr(void *kernel_src_addr)
 /**
  * @param[inout] vm pointer to a vm descriptor
  *
- * @return 0		- on success
- * @return -EINVAL	- on invalid parameters
+ * @retval 0 on success
+ * @retval -EINVAL on invalid parameters
  *
  * @pre vm != NULL
  * @pre is_vm0(vm) == true