openapi_extensions.md: fix typos and leftovers (#1128)

* openapi_extensions.md: fix typos and leftovers

dump-swagger had to substitute things in the past - these days it
just merges definitions.

* Changelog

changelogs/client_server/newsfragments/1128.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

openapi_extensions.md

@@ -12,10 +12,10 @@ To ease API design and management, the API definition is split across several
 files. Each of these files is self-contained valid OpenAPI.
 
 There is no single root file in the source tree as OpenAPI requires; this file
-can be generated by `dump_swagger.py` (also doing the substitution mentioned
+can be generated by `dump-swagger.py`. The script does not convert
-above). The script does not convert the extensions described further in this
+the extensions described further in this document (`oneOf` and parameter
-document (`oneOf` and parameter exploding) so there can be minor
+exploding) so there can be minor interoperability issues with tooling that
-interoperability issues with tooling that expects compliant Swagger.
+expects compliant Swagger.
 
 ## Extensible Query Parameters