doc: describe detail about checkambig optional argument

This is followup for patches below, which add checkambig argument to
existing function.

  - 731ced087a4b
  - 76f1ea360c7e
  - ce2d81aafbae
  - a109bf7e0dc2

mercurial/scmutil.py

@@ -379,6 +379,12 b' class abstractvfs(object):'
         return util.readlock(self.join(path))
 
     def rename(self, src, dst, checkambig=False):
+        """Rename from src to dst
+
+        checkambig argument is used with util.filestat, and is useful
+        only if destination file is guarded by any lock
+        (e.g. repo.lock or repo.wlock).
+        """
         dstpath = self.join(dst)
         oldstat = checkambig and util.filestat(dstpath)
         if oldstat and oldstat.stat:
@@ -533,7 +539,9 b' class vfs(abstractvfs):'
            file were opened multiple times, there could be unflushed data
            because the original file handle hasn't been flushed/closed yet.)
 
-        ``checkambig`` is passed to atomictempfile (valid only for writing).
+        ``checkambig`` argument is passed to atomictemplfile (valid
+        only for writing), and is useful only if target file is
+        guarded by any lock (e.g. repo.lock or repo.wlock).
         '''
         if self._audit:
             r = util.checkosfilename(path)

mercurial/util.py

@@ -1010,7 +1010,14 b' def checksignature(func):'
 
 def copyfile(src, dest, hardlink=False, copystat=False, checkambig=False):
     '''copy a file, preserving mode and optionally other stat info like
-    atime/mtime'''
+    atime/mtime
+
+    checkambig argument is used with filestat, and is useful only if
+    destination file is guarded by any lock (e.g. repo.lock or
+    repo.wlock).
+
+    copystat and checkambig should be exclusive.
+    '''
     assert not (copystat and checkambig)
     oldstat = None
     if os.path.lexists(dest):
@@ -1463,6 +1470,10 b' class atomictempfile(object):'
     the temporary copy to the original name, making the changes
     visible. If the object is destroyed without being closed, all your
     writes are discarded.
+
+    checkambig argument of constructor is used with filestat, and is
+    useful only if target file is guarded by any lock (e.g. repo.lock
+    or repo.wlock).
     '''
     def __init__(self, name, mode='w+b', createmode=None, checkambig=False):
         self.__name = name      # permanent name