Make information about data-dir env variable for tezos-client more visible

Environment (Mainnet, test network, build from source, ...)

Static tezos-client from v8.2 release.

Summary

Currently, there is no easy way to find out that the tezos-client --base-dir option can be exchanged with TEZOS_CLIENT_DIR. The only way to get information about this env variable is to run tezos-client man -v 3 or read the v8.0~rc2 changelog. On the contrary, tezos-node run --help and tezos-node config --help have a decent description of the TEZOS_NODE_DIR by default.

I think it'll be useful to display information about TEZOS_CLIENT_DIR env variable by default at least in e.g. tezos-client --help output.