copies: add more details to the documentation of mergecopies() This documentation is very helpful for any developer to understand what copytracing is and what the function does. Since this is the main function of doing copytracing, I have also included bits about copytracing in it. This additions are picked from a doc by Stash@Fb. So thanks to him. Differential Revision: https://phab.mercurial-scm.org/D409