docs (no-exports-with-element): add docs

docs/rules/no-exports-with-element.md

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+# Disallows exports alongside custom element exports (no-exports-with-element)
+
+It's possible to export multiple functions and classes in a JavaScript file.
+In the case of Custom Elements, exporting more than the Custom Element itself
+can cause confusion when importing code. It may also be a sign that there is
+too much code in a single file. If you have utility functions that the Custom
+Element depends on, it might be worth splitting this out into a separate file.
+
+## Rule Details
+
+This rule disallows exports (other than the element class and event subclasses)
+in a file with a Custom Element.
+
+👎 Examples of **incorrect** code for this rule:
+
+```js
+// foo-bar-element.js
+export class FooBarElement extends HTMLElement {
+  // ...
+}
+
+export function myHelper() {
+  // ...
+}
+```
+
+👍 Examples of **correct** code for this rule:
+
+```js
+// foo-bar-element.js
+import {myHelper} from './helpers.js'
+export class FooBarElement extends HTMLElement {
+  // ...
+}
+```
+
+```js
+// foo-bar-element.js
+import {myHelper} from './helpers.js'
+export class FooReadyEvent extends Event {
+  // ...
+}
+
+export class FooBarElement extends HTMLElement {
+  // ...
+}
+```
+
+```js
+// helpers.js
+export function myHelper() {
+  // ...
+}
+```
+
+## When Not To Use It
+
+If you intentionally want multiple exports in a single file then you can disable this rule.