doc: apply sentence-consistently in C++ style guide

Use sentence case for headers consistently. This makes the document
internally consistent and also aligns it with our docs generally and the
style guide we use for docs.

doc/guides/cpp-style-guide.md

@@ -1,11 +1,11 @@
-# C++ Style Guide
+# C++ style guide
 
 See also the [C++ codebase README](../../src/README.md) for C++ idioms in the
 Node.js codebase not related to stylistic issues.
 
-## Table of Contents
+## Table of contents
 
-* [Guides and References](#guides-and-references)
+* [Guides and references](#guides-and-references)
 * [Formatting](#formatting)
   * [Left-leaning (C++ style) asterisks for pointer declarations](#left-leaning-c-style-asterisks-for-pointer-declarations)
   * [C++ style comments](#c-style-comments)
@@ -18,11 +18,11 @@ Node.js codebase not related to stylistic issues.
   * [`snake_case_` for private class fields](#snake_case_-for-private-class-fields)
   * [`snake_case` for C-like structs](#snake_case-for-c-like-structs)
   * [Space after `template`](#space-after-template)
-* [Memory Management](#memory-management)
+* [Memory management](#memory-management)
   * [Memory allocation](#memory-allocation)
   * [Use `nullptr` instead of `NULL` or `0`](#use-nullptr-instead-of-null-or-0)
   * [Use explicit pointer comparisons](#use-explicit-pointer-comparisons)
-  * [Ownership and Smart Pointers](#ownership-and-smart-pointers)
+  * [Ownership and smart pointers](#ownership-and-smart-pointers)
   * [Avoid non-const references](#avoid-non-const-references)
   * [Use AliasedBuffers to manipulate TypedArrays](#use-aliasedbuffers-to-manipulate-typedarrays)
 * [Others](#others)
@@ -32,7 +32,7 @@ Node.js codebase not related to stylistic issues.
   * [Avoid throwing JavaScript errors in C++ methods](#avoid-throwing-javascript-errors-in-c)
     * [Avoid throwing JavaScript errors in nested C++ methods](#avoid-throwing-javascript-errors-in-nested-c-methods)
 
-## Guides and References
+## Guides and references
 
 The Node.js C++ codebase strives to be consistent in its use of language
 features and idioms, as well as have some specific guidelines for the use of
@@ -191,7 +191,7 @@ class FancyContainer {
 }
 ```
 
-## Memory Management
+## Memory management
 
 ### Memory allocation
 
@@ -208,7 +208,7 @@ Use explicit comparisons to `nullptr` when testing pointers, i.e.
 `if (foo == nullptr)` instead of `if (foo)` and
 `foo != nullptr` instead of `!foo`.
 
-### Ownership and Smart Pointers
+### Ownership and smart pointers
 
 * [R.20][]: Use `std::unique_ptr` or `std::shared_ptr` to represent ownership
 * [R.21][]: Prefer `unique_ptr` over `shared_ptr` unless you need to share