docs: prefer placing shellenv command in rcfile rather than shell profile on Linux

[XuehaiPan]

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This is a follow-up PR for Homebrew/install#809 for document update.

• Inaccurate post-install instructions install#808
• install.sh: prefer placing shellenv command in rcfile rather than shell profile on Linux install#809

[MikeMcQuaid]

Thanks again @XuehaiPan! Again will wait for another @Homebrew/maintainers to 👍🏻 before I merge.

[Bo98]

(Further discussion since has been happening Homebrew/install#808 - I left a comment there)

[MikeMcQuaid]

• Inaccurate post-install instructions 

Now this was merged @XuehaiPan could you adjust this PR accordingly? Thanks!

[MikeMcQuaid]

Great work, thanks @XuehaiPan!