patch 8.0.1451: difficult to set the python home directories properly

Problem:    It is difficult to set the python home directory properly for
            Python 2.7 and 3.5 since both use $PYTHONHOME.
Solution:   Add the 'pythonhome' and 'pythonthreehome' options. (Kazuki
            Sakamoto, closes #1266)
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index 06ae663..516a1da 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -839,7 +839,9 @@
 'prompt'	  'prompt'  enable prompt in Ex mode
 'pumheight'	  'ph'	    maximum height of the popup menu
 'pythondll'		    name of the Python 2 dynamic library
+'pythonhome'		    name of the Python 2 home directory
 'pythonthreedll'	    name of the Python 3 dynamic library
+'pythonthreehome'	    name of the Python 3 home directory
 'pyxversion'	  'pyx'	    Python version used for pyx* commands
 'quoteescape'	  'qe'	    escape characters used in a string
 'readonly'	  'ro'	    disallow writing the buffer