Update docs/how-to/provide-your-passwords.md

Provide an explanation of encryption_passcommand.
Also, adjust headers for consistency.

docs/how-to/provide-your-passwords.md

@@ -5,7 +5,7 @@ eleventyNavigation:
   parent: How-to guides
   order: 2
 ---
-## Environment variable interpolation
+## Providing passwords and secrets to borgmatic
 
 If you want to use a Borg repository passphrase or database passwords with
 borgmatic, you can set them directly in your borgmatic configuration file,
@@ -13,6 +13,18 @@ treating those secrets like any other option value. But if you'd rather store
 them outside of borgmatic, whether for convenience or security reasons, read
 on.
 
+### Delegating to a another application
+
+borgmatic supports calling another application such as a password manager to 
+obtain the borg passphrase to a repository.
+
+For example, to ask password manager to provide the passphrase:
+```yaml
+encryption_passcommand: pass path/to/borg-repokey
+```
+
+### Environment variable interpolation
+
 <span class="minilink minilink-addedin">New in version 1.6.4</span> borgmatic
 supports interpolating arbitrary environment variables directly into option
 values in your configuration file. That means you can instruct borgmatic to
@@ -52,7 +64,7 @@ This uses the `MY_DATABASE_PASSWORD` environment variable as your database
 password.
 
 
-### Interpolation defaults
+#### Interpolation defaults
 
 If you'd like to set a default for your environment variables, you can do so with the following syntax:
 
@@ -65,7 +77,7 @@ environment variable is not set. Without a default, if the environment
 variable doesn't exist, borgmatic will error.
 
 
-### Disabling interpolation
+#### Disabling interpolation
 
 To disable this environment variable interpolation feature entirely, you can
 pass the `--no-environment-interpolation` flag on the command-line.