gptel: Make gptel-backend customizable

* gptel.el (gptel-backend): Turn `gptel-backend` into a defcustom
so it can be used with setopt.  Fix #167.

* gptel-openai.el (gptel-make-openai): Improve docstring.

gptel-openai.el

@@ -118,7 +118,7 @@
           (host "api.openai.com")
           (protocol "https")
           (endpoint "/v1/chat/completions"))
-  "Register a ChatGPT backend for gptel with NAME.
+  "Register an OpenAI API-compatible backend for gptel with NAME.
 
 Keyword arguments:
 

gptel.el

@@ -417,7 +417,30 @@ with differing settings.")
    :stream t
    :models '("gpt-3.5-turbo" "gpt-3.5-turbo-16k" "gpt-4" "gpt-4-1106-preview")))
 
-(defvar-local gptel-backend gptel--openai)
+(defcustom gptel-backend gptel--openai
+  "LLM backend to use.
+
+This is the default \"backend\", an object of type
+`gptel-backend' containing connection, authentication and model
+information.
+
+A backend for ChatGPT is pre-defined by gptel.  Backends for
+other LLM providers (local or remote) may be constructed using
+one of the available backend creation functions:
+- `gptel-make-openai'
+- `gptel-make-azure'
+- `gptel-make-ollama'
+- `gptel-make-gpt4all'
+- `gptel-make-gemini'
+See their documentation for more information and the package
+README for examples."
+  :local t
+  :safe #'always
+  :group 'gptel
+  :type `(choice
+          (const :tag "ChatGPT" ,gptel--openai)
+          (restricted-sexp :match-alternatives (gptel-backend-p 'nil)
+           :tag "Other backend")))
 
 (defvar-local gptel--bounds nil)
 (put 'gptel--bounds 'safe-local-variable #'always)