[wp-docs] plugins page could use a touch-up

[Jane Wells]

http://codex.wordpress.org/Managing_Plugins

Is still focused on manual uploading and updating of plugins, should 
give equal time and prioritize within-dashboard installation and 
updating since it's more common now.

Would also be good to have a bit under troubleshooting that suggests 
turning off all plugins and re-activating one at a time to see which one 
is causing the break.
j