doc: document nullptr comparisons in style guide

This documents existing practices.

PR-URL: #23805
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

CPP_STYLE_GUIDE.md

@@ -18,6 +18,7 @@
 * [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)
   * [Avoid non-const references](#avoid-non-const-references)
 * [Others](#others)
@@ -195,6 +196,12 @@ class FancyContainer {
 
 Further reading in the [C++ Core Guidelines][ES.47].
 
+### Use explicit pointer comparisons
+
+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
 
 * [R.20]: Use `std::unique_ptr` or `std::shared_ptr` to represent ownership