docs: add more information for example values

[jcstein]

./celestia-server --enable-rpc --rpc-addr $RPC_ADDR \
      --rpc-port $RPC_PORT --celestia.auth-token $AUTH_TOKEN \
      --celestia.gas-price $GAS_PRICE \
      --celestia.gas-multiplier $GAS_MULTIPLIER \
      --celestia.namespace-id $NAMESPACEID \
      --celestia.rpc $CELESTIA_NODE_ENDPOINT \
      --celestia.keyname $KEYNAME

While these are documented in the menu, I got mixed up between rpc-addr and celestia.rpc, and also don't know what good default/example values are for:

• rpc-port - "HTTP-RPC server listening port (default 9876)" - what is this for? celestia-node?
• rpc-addr - "HTTP-RPC server listening interface (default "localhost")" - what is this for, specifically? how does it relate to celestia-node and nitro?
• celestia.gas-price - what is a good example value? is this required?
• celestia.gas-multiplier - what is a good example value? is this required?
• celestia.namespace-id - is this in hex? how many bytes are required? what is max length for integration?
• celestia.keyname - just wanna check: does this work?

[jcstein]

can't even use help menu after building on v0.2.0

./celestia-server --help
zsh: killed     ./celestia-server --help