improve doc how to install oracle client to use with agent2

Many zabbix user who try to use official solution have troubles. Items get unsupported with error:

Connection failed: ORA-00000: DPI-1047: Cannot locate a 64-bit Oracle Client library: "libclntsh.so: cannot open shared object file: No such file or directory". See https://oracle.github.io/odpi/doc/installation.html#linux for help.

It happens because by default oracle client is installed by default on a path which is not a part where dynamic linker is searching libraries.

I'm very sure it must be documented in zabbix documentation how to configure that.