docs(helm,ivy,vertico,ido): add incompatibility checks

These modules tend to conflict if more than one of them are enabled at
once. More systematic compatibility tests are in the works, but for now
this will do.

modules/completion/helm/doctor.el

@@ -0,0 +1,7 @@
+;; -*- lexical-binding: t; no-byte-compile: t; -*-
+;;; completion/helm/doctor.el
+
+(dolist (module '(ivy ido vertico))
+  (when (doom-module-p :completion module)
+    (error! "This module is incompatible with :completion %s; disable one or the other"
+            module)))

modules/completion/ido/doctor.el

@@ -0,0 +1,7 @@
+;; -*- lexical-binding: t; no-byte-compile: t; -*-
+;;; completion/ido/doctor.el
+
+(dolist (module '(helm ivy vertico))
+  (when (doom-module-p :completion module)
+    (error! "This module is incompatible with :completion %s; disable one or the other"
+            module)))

modules/completion/ivy/doctor.el

@@ -1,3 +1,7 @@
 ;; -*- lexical-binding: t; no-byte-compile: t; -*-
 ;;; completion/ivy/doctor.el
 
+(dolist (module '(helm ido vertico))
+  (when (doom-module-p :completion module)
+    (error! "This module is incompatible with :completion %s; disable one or the other"
+            module)))

modules/completion/vertico/doctor.el

@@ -1,4 +1,10 @@
-;;; completion/vertico/doctor.el -*- lexical-binding: t; -*-
+;; -*- lexical-binding: t; no-byte-compile: t; -*-
+;;; completion/vertico/doctor.el
+
+(dolist (module '(ivy helm ido))
+  (when (doom-module-p :completion module)
+    (error! "This module is incompatible with :completion %s; disable one or the other"
+            module)))
 
 (when (require 'consult nil t)
   ;; FIXME: This throws an error if grep is missing.