Help: Fix CMAKE_HOST_SYSTEM_PROCESSOR use of uname

Currently the document says that `CMAKE_HOST_SYSTEM_PROCESSOR` is the
output of `uname -p` if a system supports `uname`.  Update the
documented behavior to match `CMakeDetermineSystem.cmake` code.

Fixes: #20004

Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst

@@ -3,6 +3,13 @@ CMAKE_HOST_SYSTEM_PROCESSOR
 
 The name of the CPU CMake is running on.
 
-On systems that support ``uname``, this variable is set to the output of
-``uname -p``.  On Windows it is set to the value of the environment variable
-``PROCESSOR_ARCHITECTURE``.
+On Windows, this variable is set to the value of the environment variable
+``PROCESSOR_ARCHITECTURE``. On systems that support ``uname``, this variable is
+set to the output of:
+
+- ``uname -m`` on GNU, Linux, Cygwin, Darwin, Android, or
+- ``arch`` on OpenBSD, or
+- on other systems,
+
+  * ``uname -p`` if its exit code is nonzero, or
+  * ``uname -m`` otherwise.