copy module "dest" parameter is not only absolute (#50974)

We should explain the danger of using relative paths, but we should not
say they aren't allowed. Relative paths work and are useful, but the
caveat is that the starting point can't be guaranteed by ansible. This
commit adds that note, and removes the claim that only absolute paths
are accepted.

lib/ansible/modules/files/copy.py

@@ -38,9 +38,10 @@ options:
     version_added: '1.1'
   dest:
     description:
-    - Remote absolute path where the file should be copied to.
+    - Remote path where the file should be copied to.
     - If I(src) is a directory, this must be a directory too.
     - If I(dest) is a non-existent path and if either I(dest) ends with "/" or I(src) is a directory, I(dest) is created.
+    - If I(dest) is a relative path, the starting directory is determined by the remote host.
     - If I(src) and I(dest) are files, the parent directory of I(dest) is not created and the task fails if it does not already exist.
     type: path
     required: yes