Document xolox#session#save_cmd function as part of public API

[cpixl]

This function is exactly what is executed when the user command SaveSession is run. As the xolox#session#save_session by it's own doesn't save the file to disk (and doesn't do the checking, locking or last session persistance done inside xolox#session#save_cmd), it could be properly documented too because it is very convenient in some situations.

Currently I am manually integrating this plugin with vim-project, and the callback function will run this function to create a session for each project whenever some is opened. This function is useful in this situation, and probably there are a many other cases where this can come in handy.