docs: add libver example strings

Author: wkliao

src/pnetcdf/__init__.py

@@ -17,7 +17,7 @@ def libver():
     """
     libver()
 
-    :return: PnetCDF-Python version string.
+    :return: The PnetCDF-Python version string, for example "1.0.0".
     :rtype: str
     """
     return __version__

src/pnetcdf/_utils.pyx

@@ -799,7 +799,8 @@ cpdef inq_clibvers():
     library used to build this PnetCDF-Python module, and when the PnetCDF-C
     library was built.
 
-    :return: a string about PnetCDF-C library
+    :return: A string about PnetCDF-C library, for example, "1.13.0 of March 29, 2024".
+
     :rtype: str
     """
     ver_str = (<char *>ncmpi_inq_libvers()).decode('ascii')