Issue #12326: document the recommended idiom for checking sys.platform on Unix systems.

Also, point to the various alternatives.

Doc/library/os.rst

@@ -61,6 +61,13 @@ Notes on the availability of these functions:
    names have currently been registered: ``'posix'``, ``'nt'``, ``'mac'``,
    ``'os2'``, ``'ce'``, ``'java'``.
 
+   .. seealso::
+      :attr:`sys.platform` has a finer granularity.  :func:`os.uname` gives
+      system-dependent version information.
+
+      The :mod:`platform` module provides detailed checks for the
+      system's identity.
+
 
 .. _os-filenames:
 

Doc/library/sys.rst

@@ -714,6 +714,12 @@ always available.
    For Unix systems, this is the lowercased OS name as returned by ``uname -s``
    with the first part of the version as returned by ``uname -r`` appended,
    e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
+   Unless you want to test for a specific system version, it is therefore
+   recommended to use the following idiom::
+
+      if sys.platform.startswith('linux'):
+          # Linux-specific code here...
+
    For other systems, the values are:
 
    ================ ===========================
@@ -726,6 +732,12 @@ always available.
    OS/2 EMX         ``'os2emx'``
    ================ ===========================
 
+   .. seealso::
+      :attr:`os.name` has a coarser granularity.  :func:`os.uname` gives
+      system-dependent version information.
+
+      The :mod:`platform` module provides detailed checks for the
+      system's identity.
 
 .. data:: prefix