change NB_SUPPRESS_WARNINGS documentation as suggested

docs/api_cmake.rst

@@ -84,8 +84,12 @@ The high-level interface consists of just one CMake command:
           as a shared library for use in projects that consist of multiple
           extensions.
       * - ``NB_SUPPRESS_WARNINGS``
-        - Mark nanobind's include directories as system includes, suppressing
-          warnings generated from nanobind's and Python's header files.
+        - Mark the include directories of nanobind and Python as
+          [SYSTEM](https://cmake.org/cmake/help/latest/command/include_directories.html)
+          include directories, which suppresses any potential warning messages
+          originating there. This is mainly of relevance if your project artificially
+          raises the warning level via flags like `-pedantic`, ``-Wcast-qual``,
+          ``-Wsign-conversion``.
       * - ``PROTECT_STACK``
         - Don't remove stack smashing-related protections.
       * - ``LTO``