Update source/reference/command/compact.txt

Changed the note about the status change for a compact to be a warning 
and altered the case of the states to make it more in line with what 
is actually displayed in rs.status() and added a link to the state 
definitions for more info.
DOCS-937: fixing error page processing
merge: DOCS-936
DOCS-931: reindex propogation
DOCS-915 clarifying example