Update docs with dual-pass interp info

Signed-off-by: Kira Boyle <kirab@vmware.com>
Co-authored-by: Kira Boyle <kirab@vmware.com>

docs/interpolate/README.md

@@ -47,15 +47,15 @@ Global Flags:
 ```
 
 ## Interpolation
-
 Given a template file with a variable reference:
 
 ```yaml
 # config.yml
 key: ((variable_name))
 ```
 
-Values can be provided from a separate variables yaml file (`--vars-file`) or from environment variables (`--vars-env`).
+Values can be provided from a separate variables yaml file (`--vars-file`)
+or from environment variables (`--vars-env`).
 
 To load variables from a file use the `--vars-file` flag.
 
@@ -81,4 +81,52 @@ OM_VAR_variable_name=some_value om interpolate \
 
 The interpolation support is inspired by similar features in BOSH. You can
 [refer to the BOSH documentation](https://bosh.io/docs/cli-int/) for details on how interpolation
-is performed.
+is performed.
+
+One significant difference from similar features in BOSH
+and Concourse `fly` is that `om` performs _dual-pass_ interpolation.
+That is, the template is interpolated (including applying ops files) once,
+and then the output of that interpolation is interpolated again,
+using the same arguments (except for Ops Files,
+which are not necessarily idempotent).
+
+This allows the use of _mapping variables_,
+variables that contain a value that is a variable in turn.
+Mapping variables are useful for mapping
+between programmatically-generated variable names
+such as those created by `om config-template` and `om staged-director-config`,
+and credentials that may be used in multiple places,
+such as database passwords.
+
+Such config might look something like this:
+
+```
+cc:
+  database-password: ((cc_database_password))
+uaa:
+  database-password: ((uaa_database_password))
+```
+
+In such cases, a vars-file like this can encode the relationship:
+
+```
+---
+cc_database_password: ((sql_password))
+uaa_database_password: ((sql_password))
+```
+
+Assuming the value of `sql_password` is available to the interpolation,
+it will be present in the final output,
+like so:
+
+```
+cc:
+  database-password: actualsqlpasswordverysecure
+uaa:
+  database-password: actualsqlpasswordverysecure
+```
+
+This feature generally shouldn't interfere with interpolation
+as it would normally work in BOSH.
+If you encounter any situation where this difference is an unwelcome surprise,
+please open an issue; we were unable to think of any.

docsgenerator/templates/interpolate/ADDITIONAL_INFO.md

@@ -1,13 +1,13 @@
 ## Interpolation
-
 Given a template file with a variable reference:
 
 ```yaml
 # config.yml
 key: ((variable_name))
 ```
 
-Values can be provided from a separate variables yaml file (`--vars-file`) or from environment variables (`--vars-env`).
+Values can be provided from a separate variables yaml file (`--vars-file`)
+or from environment variables (`--vars-env`).
 
 To load variables from a file use the `--vars-file` flag.
 
@@ -33,4 +33,52 @@ OM_VAR_variable_name=some_value om interpolate \
 
 The interpolation support is inspired by similar features in BOSH. You can
 [refer to the BOSH documentation](https://bosh.io/docs/cli-int/) for details on how interpolation
-is performed.
+is performed.
+
+One significant difference from similar features in BOSH
+and Concourse `fly` is that `om` performs _dual-pass_ interpolation.
+That is, the template is interpolated (including applying ops files) once,
+and then the output of that interpolation is interpolated again,
+using the same arguments (except for Ops Files,
+which are not necessarily idempotent).
+
+This allows the use of _mapping variables_,
+variables that contain a value that is a variable in turn.
+Mapping variables are useful for mapping
+between programmatically-generated variable names
+such as those created by `om config-template` and `om staged-director-config`,
+and credentials that may be used in multiple places,
+such as database passwords.
+
+Such config might look something like this:
+
+```
+cc:
+  database-password: ((cc_database_password))
+uaa:
+  database-password: ((uaa_database_password))
+```
+
+In such cases, a vars-file like this can encode the relationship:
+
+```
+---
+cc_database_password: ((sql_password))
+uaa_database_password: ((sql_password))
+```
+
+Assuming the value of `sql_password` is available to the interpolation,
+it will be present in the final output,
+like so:
+
+```
+cc:
+  database-password: actualsqlpasswordverysecure
+uaa:
+  database-password: actualsqlpasswordverysecure
+```
+
+This feature generally shouldn't interfere with interpolation
+as it would normally work in BOSH.
+If you encounter any situation where this difference is an unwelcome surprise,
+please open an issue; we were unable to think of any.