[RFAv2 0/2] Make first and last lines of 'command help documentation' consistent + add a test

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

This patch series ensures the help doc of commands respects 2 invariants.

Patch 1 changes the code and tests so that invariants are respected;
Patch 2 implements a selftest to verify the invariants.

This is the second version of the change.

Compared to the first version, the changes are handling the comments
given by Tom and Pedro.

The changes are:
  * The test to verify the invariants is now a separate patch,
    and is implemented as a selftest, as discussed on irc.
  * Some updates to the help:
    help delete: removed "or auto-display expressions"
                 removed that unset is an alias for delete, as unset
                 is not an alias of delete.
    help interpreter-exec: after confirming that effectively
       several commands can be given, doc updated to clarify this.
    help substitute-path: replaced "source files names directories"
       by "source directories" (same wording as in the user manual).