Improve env vars docs clarity

[infomiho]

Based on this series of messages: https://ptb.discord.com/channels/686873244791210014/812664118887841813/1341866176766017587

It's confusing that we are saying "Use the env object" without letting people know that they need to implement validations for each env vars they want to have available on the env object.

We mention user env validation under the title "Custom Env Var Validations" which reads like "something extra to do" and not "you need to define validations for your env vars if you want to use the env object".

We should make it extra clear that that's what's required if you want to use the env object from wasp/server or wasp/client for your env vars. Also, we should make it obvious that the platform primitives of process.env and import.meta.env are still functional if they don't want to validate their env vars.

[yalcinos]

I just saw this issue and i also agree, I just opened one question on discord before saw this issue;
https://discord.com/channels/686873244791210014/1050789347567349791/threads/1343311829081788516