docs(user-guide): cover correct $PATH configuration for proxies

[rami3l]

As a prerequisite of merging my Homebrew/homebrew-core#218316 (and thus addressing #1236 (comment)), this addition has been suggested by @MikeMcQuaid (the current lead of homebrew) in Homebrew/homebrew-core#218316 (comment):

the actual instructions should be in the rustup formula and/or the official Rust documentation (and provide a link to there instead).

As-is the existing text is:

• far too long (the longer the text, the less likely any of it is to be read)
• unclear on what should be added where; there's no specifics

[MikeMcQuaid]

Thanks for this! One suggestion of how you could potentially make it easier to debug this.

[ChrisDenton]

Looks clear to me, thanks!