docs: configure-v3 (#4214)

Before the `configure-v3` command didn't show up in Makefile help

(cherry picked from commit 315aa86)

Makefile

@@ -323,6 +323,7 @@ disable-mptcp:
 CONFIG_FILE ?= ${HOME}/.celestia-app/config/config.toml
 SEND_RECV_RATE ?= 10485760  # 10 MiB
 
+## configure-v3: Modifies config file in-place to conform to v3.x recommendations.
 configure-v3:
 	@echo "Using config file at: $(CONFIG_FILE)"
 	@if [ "$$(uname)" = "Darwin" ]; then \