binman: Correct the documentation for u-boot-spl-bss-pad

The documentation for this entry indicates that the SPL binary is included
along with the padding. It is not, so update it to correct the error.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 3fbc06d..5c6663e 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -960,13 +960,16 @@
 Properties / Entry arguments:
     None
 
-This is similar to u_boot_spl except that padding is added after the SPL
-binary to cover the BSS (Block Started by Symbol) region. This region holds
-the various used by SPL. It is set to 0 by SPL when it starts up. If you
-want to append data to the SPL image (such as a device tree file), you must
-pad out the BSS region to avoid the data overlapping with U-Boot variables.
-This entry is useful in that case. It automatically pads out the entry size
-to cover both the code, data and BSS.
+This holds the padding added after the SPL binary to cover the BSS (Block
+Started by Symbol) region. This region holds the various variables used by
+SPL. It is set to 0 by SPL when it starts up. If you want to append data to
+the SPL image (such as a device tree file), you must pad out the BSS region
+to avoid the data overlapping with U-Boot variables. This entry is useful in
+that case. It automatically pads out the entry size to cover both the code,
+data and BSS.
+
+The contents of this entry will a certain number of zero bytes, determined
+by __bss_size
 
 The ELF file 'spl/u-boot-spl' must also be available for this to work, since
 binman uses that to look up the BSS address.