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

mercurial/copies.py

@@ -304,6 +304,28 b' def _combinecopies(copyfrom, copyto, fin'
 
 def mergecopies(repo, c1, c2, base):
     """
+    The basic algorithm for copytracing. Copytracing is used in commands like
+    rebase, merge, unshelve, etc to merge files that were moved/ copied in one
+    merge parent and modified in another. For example:
+
+    o          ---> 4 another commit
+    |
+    |   o      ---> 3 commit that modifies a.txt
+    |  /
+    o /        ---> 2 commit that moves a.txt to b.txt
+    |/
+    o          ---> 1 merge base
+
+    If we try to rebase revision 3 on revision 4, since there is no a.txt in
+    revision 4, and if user have copytrace disabled, we prints the following
+    message:
+
+    ```other changed <file> which local deleted```
+
+    If copytrace is enabled, this function finds all the new files that were
+    added from merge base up to the top commit (here 4), and for each file it
+    checks if this file was copied from another file (a.txt in the above case).
+
     Find moves and copies between context c1 and c2 that are relevant
     for merging. 'base' will be used as the merge base.