[doc] The auto replacement of "process.env.NODE_ENV" could be explained more clearly in doc

[ZYinMD]

Describe the bug

It's fine that the string "process.env.NODE_ENV" is auto replaced, but some of its unexpected behavior isn't mentioned in the doc, and causes some surprises. I think it'd be great to explain it a bit more on this page.

Reproduction

alert("process.env.NODE_ENV"); // SyntaxError: missing ) after argument list

System Info

System:
    OS: Windows 10 10.0.19043
    CPU: (8) x64 Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
    Memory: 10.74 GB / 22.95 GB
  Binaries:
    Node: 14.17.4 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.5 - ~\scoop\apps\yarn\current\Yarn\bin\yarn.CMD
    npm: 6.14.14 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 92.0.4515.159
    Edge: Spartan (44.19041.1023.0), Chromium (92.0.902.84)
    Internet Explorer: 11.0.19041.906

Used Package Manager

yarn

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/vue-next instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[lovetingyuan]

What if I do want to replace process.env.NODE_ENV, such as using lib build mode. @patak-js