efi_memory: refine overlap_only_ram description

Refine text for overlap_only_ram description to
match to what exactly flag does and aling description
with other functions.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_loader/efi_memory.c b/lib/efi_loader/efi_memory.c
index 7be756e..11e7553 100644
--- a/lib/efi_loader/efi_memory.c
+++ b/lib/efi_loader/efi_memory.c
@@ -235,7 +235,7 @@
  * @start:		start address, must be a multiple of EFI_PAGE_SIZE
  * @pages:		number of pages to add
  * @memory_type:	type of memory added
- * @overlap_only_ram:	the memory area must overlap existing
+ * @overlap_only_ram:	region may only overlap RAM
  * Return:		status code
  */
 static efi_status_t efi_add_memory_map_pg(u64 start, u64 pages,