[Docs] forbid-foreign-prop-types: document allowInPropTypes option

Fixes #1815

CHANGELOG.md

@@ -28,6 +28,7 @@ This change log adheres to standards from [Keep a CHANGELOG](https://keepachange
 * [Docs] [`jsx-no-target-blank`]: Improve readme ([#3169][] @apepper)
 * [Docs] [`display-name`]: improve examples ([#3189][] @golopot)
 * [Refactor] [`no-invalid-html-attribute`]: sort HTML_ELEMENTS and messages ([#3182][] @Primajin)
+* [Docs] [`forbid-foreign-prop-types`]: document `allowInPropTypes` option ([#1815][] @ljharb)
 
 [#3195]: https://github.com/yannickcr/eslint-plugin-react/pull/3195
 [#3191]: https://github.com/yannickcr/eslint-plugin-react/pull/3191
@@ -43,6 +44,7 @@ This change log adheres to standards from [Keep a CHANGELOG](https://keepachange
 [#3133]: https://github.com/yannickcr/eslint-plugin-react/pull/3133
 [#2921]: https://github.com/yannickcr/eslint-plugin-react/pull/2921
 [#2753]: https://github.com/yannickcr/eslint-plugin-react/pull/2753
+[#1815]: https://github.com/yannickcr/eslint-plugin-react/issues/1815
 [#1754]: https://github.com/yannickcr/eslint-plugin-react/issues/1754
 [#1046]: https://github.com/yannickcr/eslint-plugin-react/issues/1046
 [#620]: https://github.com/yannickcr/eslint-plugin-react/pull/620

docs/rules/forbid-foreign-prop-types.md

@@ -2,7 +2,7 @@
 
 This rule forbids using another component's prop types unless they are explicitly imported/exported. This allows people who want to use [babel-plugin-transform-react-remove-prop-types](https://github.com/oliviertassinari/babel-plugin-transform-react-remove-prop-types) to remove propTypes from their components in production builds, to do so safely.
 
-In order to ensure that imports are explicitly exported it is recommended to use the ["named" rule in eslint-plugin-import](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/named.md) in conjunction with this rule.
+In order to ensure that imports are explicitly exported it is recommended to use the ["named" rule in eslint-plugin-import](https://github.com/benmosher/eslint-plugin-import/blob/HEAD/docs/rules/named.md) in conjunction with this rule.
 
 ## Rule Details
 
@@ -25,6 +25,18 @@ Examples of **correct** code for this rule:
 import SomeComponent, {propTypes as someComponentPropTypes} from './SomeComponent';
 ```
 
+## Rule Options
+
+```js
+...
+"react/forbid-foreign-prop-types": [<enabled>, { "allowInPropTypes": [<boolean>] }]
+...
+```
+
+### `allowInPropTypes`
+
+If `true`, the rule will not warn on foreign propTypes usage inside a propTypes declaration.
+
 ## When Not To Use It
 
 This rule aims to make a certain production optimization, removing prop types, less prone to error. This rule may not be relevant to you if you do not wish to make use of this optimization.