Restructure and fix and improve docs and manpages.

docs/man/cdist-remote-exec-copy.rst

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+Remote exec and copy commands
+=============================
+
+Cdist interacts with the target host in two ways:
+
+- it executes code (__remote_exec)
+- and it copies files (__remote_copy)
+
+By default this is accomplished with ssh and scp respectively.
+The default implementations used by cdist are::
+
+    __remote_exec: ssh -o User=root -q
+    __remote_copy: scp -o User=root -q
+
+The user can override these defaults by providing custom implementations and
+passing them to cdist with the --remote-exec and/or --remote-copy arguments.
+
+For __remote_exec, the custom implementation must behave as if it where ssh.
+For __remote_copy, it must behave like scp.
+
+With this simple interface the user can take total control of how cdist
+interacts with the target when required, while the default implementation 
+remains as simple as possible.