Formula cookbook: document with_env

The other syntax for setting `ENV` was added in #9013,
but this form remained undocumented.

Fixes #8648.

Library/Homebrew/utils.rb

@@ -488,6 +488,7 @@ module Kernel
   # @note This method is *not* thread-safe - other threads
   #   which happen to be scheduled during the block will also
   #   see these environment variables.
+  # @api public
   def with_env(hash)
     old_values = {}
     begin

docs/Formula-Cookbook.md

@@ -757,7 +757,7 @@ Firstly, the overall environment in which Homebrew runs is filtered to avoid env
 
 The second level of filtering removes sensitive environment variables (such as credentials like keys, passwords or tokens) to avoid malicious subprocesses obtaining them (<https://github.com/Homebrew/brew/pull/2524>). This has the effect of preventing any such variables from reaching a formula's Ruby code as they are filtered before it is called. The specific implementation can be seen in the [`ENV.clear_sensitive_environment!` method](https://github.com/Homebrew/brew/blob/HEAD/Library/Homebrew/extend/ENV.rb).
 
-You can set environment variables in a formula's `install` method using `ENV["VARIABLE_NAME"] = "VALUE"`. An example can be seen in [the `gh` formula](https://github.com/Homebrew/homebrew-core/blob/fd9ad29f8e3ca9476f838ebb13794ddb7dafba00/Formula/gh.rb#L22).
+You can set environment variables in a formula's `install` method using `ENV["VARIABLE_NAME"] = "VALUE"`. An example can be seen in [the `gh` formula](https://github.com/Homebrew/homebrew-core/blob/fd9ad29f8e3ca9476f838ebb13794ddb7dafba00/Formula/gh.rb#L22). Environment variables can also be set temporarily using the `with_env` method; any variables defined in the call to that method will be restored to their original values at the end of the block. An example can be seen in [the `csound` formula](https://github.com/Homebrew/homebrew-core/blob/c3feaff8cdb578331385676620c865796cfc3388/Formula/csound.rb#L155-L157).
 
 In summary, environment variables used by a formula need to conform to these filtering rules in order to be available.