Improve `@nrwl/next:server` `customServerPath` docs

[andre-matulionis-ifood]

This adds more details on what is the customServerPath option, and how to use it.

Some of my problems were:

1. I created a server.js, that did not export a function, instead, it was a file that was run: node server.js. It showed an error that "customServer is not a function"
2. I changed server.js to export a default function. Now it did run, but it still didn't work as expected
3. I noticed a specific comment in an specific issue that showed what was the function signature, and an example on how to use it, which fixed all errors

Because the issue has a public solution but it is not documented, I added it to the documentation

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[joaowillamy]

great

[ndcunningham]

Hey! Thanks for contributing can you rebase your branch with the latest master?

[ndcunningham]

Can you rebase again please

[jaysoo]

Deprecated the feature.

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This pull request has already been merged/closed. If you experience issues related to these changes, please open a new issue referencing this pull request.