env_config: standardize wording, fix some typos

[jonchang]

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Standardizes the wording of environment variables. In particular, say "Homebrew uses this..." for variables where this value is set by default and merely refers to e.g., a standard path, and say "Homebrew will foo the bar..." for variables that change Homebrew's default behavior somehow.

Also fix several typos and spacing problems with multiline continuations.

[MikeMcQuaid]

All looks good. I'm somewhat tempted to strip the Homebrew ... prefix considering you're reading the Homebrew manpage but I'll defer to you there.

[jonchang]

• updating wording to avoid constantly repeating "Homebrew"

[jonchang]

Thanks for the review, and the whole ENV cleanup!