[docs] Revise and update the reindexing document

[sumerjabri]

Please revise and update the reinexing documentation to ensure it's still correct and works as described: http://docs.craftercms.org/en/3.0/system-administrators/reindexing-content.html

[alhambrav]

PR craftercms/docs#470

[sumerjabri]

@avasquez614 please update and assign back to me.

[avasquez614]

I made the changes, just waiting for @alhambrav to approve them.