net: tftp: Add help for CONFIG_TFTP_BLOCKSIZE

Add help message for the CONFIG_TFTP_BLOCKSIZE default value,
as explain in tftp.c before migration in commit b618b3707633
("net: Convert CONFIG_TFTP_BLOCKSIZE to Kconfig")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/net/Kconfig b/net/Kconfig
index 96bbce1..ac6d0cf 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -44,5 +44,9 @@
 	default 1468
 	help
 	  Default TFTP block size.
+	  The MTU is typically 1500 for ethernet, so a TFTP block of
+	  1468 (MTU minus eth.hdrs) provides a good throughput with
+	  almost-MTU block sizes.
+	  You can also activate CONFIG_IP_DEFRAG to set a larger block.
 
 endif   # if NET