doc/Installation: make clear the env configs are advanced settings

Co-authored-by: Kevin <[email protected]>
Co-authored-by: Mike McQuaid <[email protected]>

docs/Installation.md

@@ -15,19 +15,25 @@ The macOS `.pkg` installer also installs Homebrew to its default prefix (`/opt/h
   [Xcode](https://itunes.apple.com/us/app/xcode/id497799835) <sup>[3](#3)</sup>
 * The Bourne-again shell for installation (i.e. `bash`) <sup>[4](#4)</sup>
 
-## Git Remote Mirroring
+## Advanced Configuration
 
-You can use geolocalized Git mirrors to speed up Homebrew's installation and `brew update` by setting `HOMEBREW_BREW_GIT_REMOTE` and/or `HOMEBREW_CORE_GIT_REMOTE` in your shell environment with this script:
+The Homebrew installer offers various advanced configuration settings. **Most users can skip this section and instead follow the instructions on the [homepage](https://brew.sh)!**
+
+### Git Remote Mirroring
+
+If you have issues connecting to GitHub.com, you can use Git mirrors for Homebrew's installation and `brew update` by setting `HOMEBREW_BREW_GIT_REMOTE` and/or `HOMEBREW_CORE_GIT_REMOTE` in your shell environment with this script:
 
 ```bash
 export HOMEBREW_BREW_GIT_REMOTE="..."  # put your Git mirror of Homebrew/brew here
 export HOMEBREW_CORE_GIT_REMOTE="..."  # put your Git mirror of Homebrew/homebrew-core here
 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
 ```
 
-The default Git remote will be used if the corresponding environment variable is unset.
+The default Git remote will be used if the corresponding environment variable is unset and works best for most users.
+
+**Note:** if you set these variables you are granting these repositories the same level of trust you currently grant to Homebrew itself. You should be extremely confident that these repositories will not be compromised.
 
-## Default Tap Cloning
+### Default Tap Cloning
 
 You can instruct Homebrew to return to pre-4.0.0 behaviour by cloning the Homebrew/homebrew-core tap during installation by setting the `HOMEBREW_NO_INSTALL_FROM_API` environment variable with the following:
 
@@ -36,9 +42,9 @@ export HOMEBREW_NO_INSTALL_FROM_API=1
 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
 ```
 
-This will make Homebrew install formulae and casks from the `homebrew/core` and `homebrew/cask` taps using local checkouts of these repositories instead of Homebrew’s API.
+This will make Homebrew install formulae and casks from the `homebrew/core` and `homebrew/cask` taps using local checkouts of these repositories instead of Homebrew’s API. Unless you are a Homebrew maintainer or contributor, you should probably not globally enable this setting. It can easily be enabled later after installation should it be necessary.
 
-## Unattended installation
+### Unattended installation
 
 If you want a non-interactive run of the Homebrew installer that doesn't prompt for passwords (e.g. in automation scripts), prepend [`NONINTERACTIVE=1`](https://github.com/Homebrew/install/#install-homebrew-on-macos-or-linux) to the installation command.