Improve documentation

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/alignment.h b/library/alignment.h
index 755cde3..dedfdc9 100644
--- a/library/alignment.h
+++ b/library/alignment.h
@@ -31,6 +31,7 @@
  * be aligned.
  *
  * \param   p pointer to 4 bytes of data
+ * \return  Data at the given address
  */
 inline uint32_t mbedtls_get_unaligned_uint32( void const *p )
 {