clearly state STELLAR_CORE_URL usage in terms of tx sub enabled insta…

…nce (#274)

docs/run-platform-server/configuring.mdx

@@ -62,16 +62,16 @@ You can run none or multiple instances to serve read only API requests. Notice t
 
 ### Transaction Submission Role Instance
 
-You can run none or multiple instances to serve transaction submission requests. If you run an instance with transaction submission enabled, your deployment is required to perform the ingestion role on the same database referenced here. Horizon transaction submission depends on **live** ingestion to confirm tx submission status.
+You can run none or multiple instances to serve transaction submission requests. If you run an instance with transaction submission enabled, the horizon deployment is required to have at least one instance perform the ingestion role on the same database. Horizon transaction submission depends on this **live** ingestion taking place against the database in order to confirm tx submission status.
 
-If ingestion is planned to be done on a separate instance, add `INGEST=false` on this instance, otherwise don't include the parameter, Horizon will default to `INGEST=true` if absent.
+If ingestion is planned to be done on a separate instance, add `INGEST=false` on this instance, otherwise don't include the parameter, Horizon will default to `INGEST=true`. When a transaction submission enabled instance has `INGEST=true` effective, it will configure the related `STELLAR_CORE_URL` parameter automatically to use the internally launched captive core instance and the deployment does not need to set the configuration value explicitly.
 
-If not using the `NETWORK` variable, then must provide the `STELLAR_CORE_URL` variable.
+If setting `INGEST=false`, then **must** define the `STELLAR_CORE_URL` variable on this transaction submission enabled instance, since there will be no internally hosted captive core instance as part of ingestion available to reference, instead the `STELLAR_CORE_URL` provides the ability to define the url of a core instance http port which horizon will send transaction submissions towards.
 
 | environment variable | example                             |
 | -------------------- | ----------------------------------- |
 | `DATABASE_URL`       | postgres://localhost/horizon_pubnet |
-| `NETWORK`            | pubnet                              |
+| `STELLAR_CORE_URL`   | http://example.watcher.core:11626   |
 | `INGEST`             | false                               |
 
 ## Notes
@@ -80,10 +80,10 @@ If not using the `NETWORK` variable, then must provide the `STELLAR_CORE_URL` va
 
 - `DISABLE_TX_SUB`, optional, set as FALSE by default. Controls whether Horizon will accept HTTP requests to `/tx` API endpoint and forward to network. Refer to [Channel Accounts](../encyclopedia/channel-accounts.mdx) for some recommendations on optional client transaction submission optimizations.
 
-  - When set to FALSE, it requires **live** ingestion process to be running on the same database because Horizon depends on new ledgers from network to confirm a transaction submission status, Horizon will report startup error if it detects no **live** ingestion. Requires `NETWORK` or `STELLAR_CORE_URL` be defined for access to network.
+  - When set to FALSE, it requires **live** ingestion process to be running on the same database because Horizon depends on new ledgers from network to confirm a transaction submission status, Horizon will report startup error if it detects no **live** ingestion. Requires `INGEST=true` or `STELLAR_CORE_URL` be defined for access to a core instance.
   - When transaction submission is disabled by setting it to TRUE, Horizon will return 405 on POSTs to /tx.
 
-- `NETWORK`, optional, can be one of Stellar's public networks, 'pubnet', or 'devnet'. Triggers horizon to automatically set configurations for remaining horizon settings and generate the correct core toml/cfg settings. If you only need Horizon to connect to one of those public Stellar networks, this will take care of all related configuration.
+- `NETWORK`, optional, can be one of Stellar's public networks, 'pubnet', or 'testnet'. Triggers horizon to automatically set configurations for remaining horizon settings and generate the correct core toml/cfg settings. If you only need Horizon to connect to one of those public Stellar networks, this will take care of all related configuration.
 
 - If you want to connect Horizon to a different stellar network other than pubnet or testnet or override any of the defaults that `NETWORK` usage will initiate, the key envrionment variables that can be set are: `HISTORY_ARCHIVE_URLS`, `CAPTIVE_CORE_CONFIG_PATH`, `NETWORK_PASSPHRASE`, `CAPTIVE_CORE_STORAGE_PATH`, `STELLAR_CORE_URL`.