Updated strict-mode docs to use `UNSAFE_*` lifecycle method names (#2683)

* Updated strict-mode docs to use UNSAFE_ method names

The strict mode docs still reference the old `componentWill*` lifecycle method names, this updates them to be `UNSAFE_componentWill*`

* Tweaked docs to mention both unsafe lifecycle names

Co-authored-by: Brian Vaughn <brian.david.vaughn@gmail.com>

content/docs/strict-mode.md

@@ -87,9 +87,9 @@ The commit phase is usually very fast, but rendering can be slow. For this reaso
 
 Render phase lifecycles include the following class component methods:
 * `constructor`
-* `componentWillMount`
+* `componentWillMount` (or `UNSAFE_componentWillMount`)
-* `componentWillReceiveProps`
+* `componentWillReceiveProps` (or `UNSAFE_componentWillReceiveProps`)
-* `componentWillUpdate`
+* `componentWillUpdate` (or `UNSAFE_componentWillUpdate`)
 * `getDerivedStateFromProps`
 * `shouldComponentUpdate`
 * `render`