Add additional documentation link to environment variable feature. Rename constants section.

NEWS

@@ -4,7 +4,7 @@
  * #576: Add support for "file://" paths within "repositories" option.
  * #612: Define and use custom constants in borgmatic configuration files. See the documentation for
    more information:
-   https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#constants
+   https://torsion.org/borgmatic/docs/how-to/make-per-application-backups/#constant-interpolation
  * #618: Add support for BORG_FILES_CACHE_TTL environment variable via "borg_files_cache_ttl" option
    in borgmatic's storage configuration.
  * #623: Fix confusing message when an error occurs running actions for a configuration file.

docs/how-to/make-per-application-backups.md

@@ -257,7 +257,7 @@ that your shell may interpret.
 An alternate to command-line overrides is passing in your values via [environment variables](https://torsion.org/borgmatic/docs/how-to/provide-your-passwords/).
 
 
-## Constants
+## Constant interpolation
 
 <span class="minilink minilink-addedin">New in version 1.7.10</span> Another
 tool is borgmatic's support for defining custom constants. This is similar to
@@ -312,3 +312,6 @@ retention:
 consistency:
     prefix: bar-
 ```
+
+An alternate to constants is passing in your values via [environment
+variables](https://torsion.org/borgmatic/docs/how-to/provide-your-passwords/).