Document `sage --sync-build` in `sage --advanced`

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/14046">trac #14046</a>.

apply to $SAGE_ROOT

Reported by: kini

Component: documentation

CC: 

Report Upstream: N/A

Authors: Keshav Kini

Dependencies: 

Work issues: 

Reviewers: Jeroen Demeyer

Merged in: sage-5.7.beta4

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/14046/trac_14046-document-sync-build.patch">trac_14046-document-sync-build.patch: apply to $SAGE_ROOT</a> by kini at 20130201T03:29:19</li></ol>