docs: add preamble to low-level client section

docs/GENERATING-A-NEW-SDK.md

@@ -223,6 +223,33 @@ TODO: Point to example build/publish CI workflows
 
 ## Low-level client
 
+The following is a specification for a "low-level client" called something like `OpenFgaApi`.
+This is the basic client that OpenAPI Generator will produce. It should already deal directly
+with the serialization of requests, construction and dispatch of low-level HTTP requests, and
+the deserialization of responses.
+
+Each operation should follow common [Function signatures](#function-signatures).
+
+In addition, the low-level client should support [Configuration](#configuration) and
+[Per-request Override](#per-request-override).
+
+### Function signatures
+
+Functions of the low-level client should:
+
+* Take as input a String `storeId`
+  * `CreateStore` and `ListStores` are the only operations that do not take/require a `storeId`
+  * The `storeId` should be validated as a non-null, non-blank String
+* If applicable for the operation, take as input a modeled request `body`
+  * For example, `Check` should take a `CheckRequest` as its request `body`
+  * The `body` should at minimum be validated as non-null
+* If it is a paginated operation, it should take as optional inputs an Integer `pageSize`
+  and a String `continuationToken`
+* Take as an optional input a [`ConfigurationOverride`](#per-request-override)
+* Produce as output a promise/future that will complete with:
+  * HTTP response details. This includes the status code, headers, and raw response body
+  * If applicable for the operation, a modeled request response
+
 ### Configuration
 
 #### BaseConfiguration
@@ -342,23 +369,6 @@ ClientCredentials should be only static data, and will not be concerned with per
 calls or retrieving an access token. ClientCredentials will be resolved as part of
 [Authentication](#authentication).
 
-### Function signatures
-
-Functions of the low-level client should:
-
-* Take as input a String `storeId`
-  * `CreateStore` and `ListStores` are the only operations that do not take/require a `storeId`
-  * The `storeId` should be validated as a non-null, non-blank String
-* If applicable for the operation, take as input a modeled request `body`
-  * For example, `Check` should take a `CheckRequest` as its request `body`
-  * The `body` should at minimum be validated as non-null
-* If it is a paginated operation, it should take as optional inputs an Integer `pageSize`
-  and a String `continuationToken`
-* Take as an optional input a [`ConfigurationOverride`](#per-request-override)
-* Produce as output a promise/future that will complete with:
-  * HTTP response details. This includes the status code, headers, and raw response body
-  * If applicable for the operation, a modeled request response
-
 ### Per-request override
 
 The low-level client should allow an optional `ConfigurationOverride` as input for each of its