doc: use sentence-case for headings in docs

PR-URL: #33889
Backport-PR-URL: #33964
Refs: https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings
Refs: https://docs.microsoft.com/en-us/style-guide/capitalization
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

doc/api/addons.md

@@ -1,4 +1,4 @@
-# C++ Addons
+# C++ addons
 
 <!--introduced_in=v0.10.0-->
 <!-- type=misc -->
@@ -395,7 +395,7 @@ only the symbols exported by Node.js will be available.
 source image. Using this option, the Addon will have access to the full set of
 dependencies.
 
-### Loading Addons using `require()`
+### Loading addons using `require()`
 
 The filename extension of the compiled Addon binary is `.node` (as opposed
 to `.dll` or `.so`). The [`require()`][require] function is written to look for
@@ -410,7 +410,7 @@ there is a file `addon.js` in the same directory as the binary `addon.node`,
 then [`require('addon')`][require] will give precedence to the `addon.js` file
 and load it instead.
 
-## Native Abstractions for Node.js
+## Native abstractions for Node.js
 
 Each of the examples illustrated in this document make direct use of the
 Node.js and V8 APIs for implementing Addons. The V8 API can, and has, changed

doc/api/async_hooks.md

@@ -1,4 +1,4 @@
-# Async Hooks
+# Async hooks
 
 <!--introduced_in=v8.1.0-->
 
@@ -127,7 +127,7 @@ class MyAddedCallbacks extends MyAsyncCallbacks {
 const asyncHook = async_hooks.createHook(new MyAddedCallbacks());
 ```
 
-##### Error Handling
+##### Error handling
 
 If any `AsyncHook` callbacks throw, the application will print the stack trace
 and exit. The exit path does follow that of an uncaught exception, but
@@ -201,7 +201,7 @@ be called again until enabled.
 
 For API consistency `disable()` also returns the `AsyncHook` instance.
 
-#### Hook Callbacks
+#### Hook callbacks
 
 Key events in the lifetime of asynchronous events have been categorized into
 four areas: instantiation, before/after the callback is called, and when the
@@ -631,7 +631,7 @@ only on chained promises. That means promises not created by `then()`/`catch()`
 will not have the `before` and `after` callbacks fired on them. For more details
 see the details of the V8 [PromiseHooks][] API.
 
-## JavaScript Embedder API
+## JavaScript embedder API
 
 Library developers that handle their own asynchronous resources performing tasks
 like I/O, connection pooling, or managing callback queues may use the