Make option syntax clear and consistent

[alexandratran]

There are some inconsistencies with how we present the syntax of various CLI options in the reference. For example:

--rpc-http-tls-cipher-suite=<cipherSuiteName>[, <cipherSuiteName>...]

--bootnodes[=<enode://id@host:port>[,<enode://id@host:port>...]...]

--net-restrict=<subnet>[,<subnet>,...]

--banned-node-ids=<bannedNodeId>[,<bannedNodeId>...]...

These are some inconsistent examples of how we denote comma-separated lists, and there are still other variants. Also, we currently use square brackets to denote an optional value, but some options might require using square brackets for arrays, which could cause confusion. We should decide on a consistent and clear way to document these types of options.