patch 8.0.0251: not easy to select Python 2 or 3

Problem:    It is not so easy to write a script that works with both Python 2
            and Python 3, even when the Python code works with both.
Solution:   Add 'pyxversion', :pyx, etc. (Marc Weber, Ken Takata)
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index 4ebf999..81b2eb6 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -1440,6 +1440,10 @@
 |:python|	:py[thon]	execute Python command
 |:pydo|		:pyd[o]		execute Python command for each line
 |:pyfile|	:pyf[ile]	execute Python script file
+|:pyx|		:pyx		execute |python_x| command
+|:pythonx|	:pythonx	same as :pyx
+|:pyxdo|	:pyxd[o]	execute |python_x| command for each line
+|:pyxfile|	:pyxf[ile]	execute |python_x| script file
 |:quit|		:q[uit]		quit current window (when one window quit Vim)
 |:quitall|	:quita[ll]	quit Vim
 |:qall|		:qa[ll]		quit Vim