[clang-tools-extra] [clang-tidy][docs] improve documentation on cppcoreguidelines-narrowing-conversions (#111510) (PR #118209)

[Jonas Toth via cfe-commits]

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@@ -27,6 +27,29 @@ This check will flag:
  - All applications of binary operators with a narrowing conversions.
    For example: ``int i; i+= 0.1;``.
 
+Note that arithmetic with integer types may perform implicit conversions if the used integer types are smaller than ``int``.
+These rules are documented under `"Integral Promotion" on this cppreference.com <https://en.cppreference.com/w/cpp/language/implicit_conversion>`_
+page. The following example demonstrates this behavior and can be explored with `cppinsights.io <https://cppinsights.io/s/68553908>`_.
+
+.. code-block:: c++
+
+   // The following function definition demonstrates usage of arithmetic with integer types smaller than `int`
+   // and how the narrowing conversion happens implicitly.
+   void computation(short argument1, short argument2) {
+     // Arithmetic written by humans:
+     short result = argument1 + argument2;
+     // Arithmetic actually performed by C++:
+     short result = static_cast<short>(static_cast<int>(argument1) + static_cast<int>(argument2));
+   }
+
+   void recommended_with_gsl(short argument1, short argument2) {
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JonasToth wrote:

The coreguidelines recommend `narrow_cast<>` of course,  so i wanted it to stand out.
Solving this with `static_cast<>` is the example above. I will clarify that those are both ways to deal with warning if casting is used.

https://github.com/llvm/llvm-project/pull/118209