gdb: fix formatting for help set/show extended-prompt

gdb: fix formatting for help set/show extended-prompt

The formatting of the help text for 'help set extended-prompt' and
'help show extended-prompt' is a little off.

Here's the offending snippet:

    Substitutions are applied to VALUE to compute the real prompt.

    The currently defined substitutions are:
      \[	Begins a sequence of non-printing characters.
  \\	A backslash.
  \]	Ends a sequence of non-printing characters.
  \e	The ESC character.

Notice that the line for '\[' is indented more that the others.

Turns out this is due to how we build this help text, something which
is done in Python.  We extended a classes __doc__ string with some
dynamically generated text.

The classes doc string looks like this:

    """Set the extended prompt.

    Usage: set extended-prompt VALUE

    Substitutions are applied to VALUE to compute the real prompt.

    The currently defined substitutions are:
    """

Notice the closing """ are in a line of their own, and include some
white space just before.  It's this extra white space that's causing
the problem.

Fix the formatting issue by moving the """ to the end of the previous
line.  I then add the extra newline in at the point where the doc
string is merged with the dynamically generated text.

Now everything lines up correctly.