docs: update language-onboarding guide (#1900)

The language onboarding guide will now be the source of truth for all
container contracts. It is during language onboarding that this
information is particularily useful and it does not make sense to have
this information dupilicated.

Additionally:
- Filled in a minor gap mentioned in the old doc about logging from the
container.
- Removed a TODO as the guide already mentions global file edits.

Fixes: #1828

Author: codyoss

doc/container-contract.md

@@ -66,7 +66,8 @@ it must have a binary entrypoint that can accept the arguments passed by Librari
 
 A successful container invocation is expected to exit with a code of `0`. Any non-zero exit code will be treated as an
 error and will halt the current workflow. If a container would like to send an error message back to librarian it can do
-so by including a field in the various response files outlined below.
+so by including a field in the various response files outlined below. Additionally any logs sent to stderr/stdout will
+be surfaced to the CLI.
 
 The following sections detail the contracts for each container command.
 
@@ -79,9 +80,8 @@ The container is expected to produce up to two artifacts:
 
 * A `configure-response.json` file, which is derived from the `configure-request.json` and contains language-specific
   details. This response will be committed back to the `state.yaml` file by Librarian.
-* Any "side-configuration" files that the language may need for its libraries. These should be written to the `/input` mount, which corresponds to the `.librarian/generator-input` directory in the language repository.
-
-TODO: Global file edits
+* Any "side-configuration" files that the language may need for its libraries. These should be written to the `/input`
+  mount, which corresponds to the `.librarian/generator-input` directory in the language repository.
 
 **Contract:**