examples: improve shell session usability

- Add usage prompt when starting shell sessions

- Give all examples an uniform interface ("c") for running commands
  or starting a shell on the node.

examples/start-bash-session.sh

@@ -1,6 +1,15 @@
 # Start an interactive bash session in the current bash environment.
 
+USAGE_INFO='
+Starting shell...
+Run "c COMMAND" to execute a command on the bitcoin node
+Run "c" to start a shell session inside the node'
+
 # BASH_ENVIRONMENT contains definitions of read-only variables like 'BASHOPTS' that
-# cause warnings on importing. Suppress these warnings during bash startup.
+# cause warnings on evaluation. Suppress these warnings while sourcing.
 BASH_ENVIRONMENT=<(declare -p; declare -pf) \
-  bash --rcfile <(echo 'source $BASH_ENVIRONMENT 2>/dev/null')
+USAGE_INFO="$USAGE_INFO" \
+  bash --rcfile <(echo '
+    source $BASH_ENVIRONMENT 2>/dev/null
+    echo "$USAGE_INFO"
+  ')