Re commercialhaskell#6610 Document STACK_CONFIG and STACK_GLOBAL_CONFIG

Author: mpilgrem

doc/environment_variables.md

@@ -84,10 +84,28 @@ Stack follows the standard at http://no-color.org/. Stack checks for a
 (regardless of its value), Stack prevents the addition of control character
 sequences for color to its output.
 
+## `STACK_CONFIG`
+
+Related command: all commands that make use of Stack's
+[global YAML configuration files](yaml_configuration.md) (`config.yaml`).
+
+The environment variable `STACK_CONFIG` can be used to specify an absolute path
+to the user-specific global YAML configuration file, overriding the default.
+
+## `STACK_GLOBAL_CONFIG`
+
+Related command: all commands that make use of Stack's
+[global YAML configuration files](yaml_configuration.md) (`config.yaml`).
+
+The environment variable `STACK_GLOBAL_CONFIG` can be used to specify an
+absolute path to the system-wide global YAML configuration file, overriding the
+default.
+
 ## `STACK_ROOT`
 
-Related command: all commands that make use of Stack's global YAML configuration
-file (`config.yaml`).
+Related command: all commands that make use of Stack's
+[user-specific global YAML configuration file](yaml_configuration.md)
+(`config.yaml`).
 
 Overridden by: Stack's global
 [`--stack-root`](global_flags.md#-stack-root-option) option.
@@ -111,8 +129,9 @@ the root directory of the project or package. The relative path cannot include a
 
 ## `STACK_XDG`
 
-Related command: all commands that make use of Stack's user-specific general
-YAML configuration file (`config.yaml`).
+Related command: all commands that make use of Stack's
+[user-specific global YAML configuration file](yaml_configuration.md)
+(`config.yaml`).
 
 Overridden by: the use of Stack's `STACK_ROOT` environment variable, or the use
 of Stack's global
@@ -124,8 +143,8 @@ set to any non-empty value.
 
 ## `STACK_YAML`
 
-Related command: all commands that make use of Stack's project-level YAML
-configuration file.
+Related command: all commands that make use of Stack's
+[project-level YAML configuration file](yaml_configuration.md).
 
 Overridden by: Stack's global
 [`--stack-yaml`](global_flags.md#-stack-yaml-option) option.