Help: Update compiler-specific add_compile_options() example and prose

Avoid hard-coding a warning-as-error flag now that CMake offers a
first class abstraction for that.

Also mention some relevant generator expressions which can be used
to provide even more specific targeting with language-specific flags.

Fixes: #24200

Help/command/add_compile_options.rst

@@ -28,13 +28,16 @@ this command is in a compiler-specific conditional clause:
 .. code-block:: cmake
 
   if (MSVC)
-      # warning level 4 and all warnings as errors
-      add_compile_options(/W4 /WX)
+      # warning level 4
+      add_compile_options(/W4)
   else()
-      # lots of warnings and all warnings as errors
-      add_compile_options(-Wall -Wextra -pedantic -Werror)
+      # additional warnings
+      add_compile_options(-Wall -Wextra -Wpedantic)
   endif()
 
+To set per-language options, use the :genex:`$<COMPILE_LANGUAGE>`
+or :genex:`$<COMPILE_LANGUAGE:languages>` generator expressions.
+
 See Also
 ^^^^^^^^