Make documentation of "python" command match actual behavior

Make documentation of "python" command match actual behavior

The example in the documentation for the "python" command shows GDB
outputting instructions for how to terminate a sequence of python
commands entered from the command line.  The documentation shows that
the following two lines are being output, though this does not occur
when actually using the "python" command from GDB:

    Type python script
    End with a line saying just "end".

While display of this text might be helpful, GDB has several other
commands which also use the "end" terminator that offer no such text.
Examples include the "if" and "while" commands.  For example,

(gdb) if 1==1
 >print "a"
 >end
$1 = "a"

This seems similar to doing:

(gdb) python
 >print 23
 >end
23

If we decide that we want the "python" command to print such a message,
we should also adjust the behavior for other GDB commands which also use
"end" to terminate a command list.  I.e, if this decision is made, the
"if" and "while" commands ought to also print similar messages.

So, for the moment anyway, this commit adjusts the documentation of the
python command to match its implementation.

This patch was taken from a larger body of work originating from the
Archer project.  I haven't been able to determine its original author,
though I did find a commit log from Jan Kratochvil (in the Archer
repository) which suggests that the change had originally been made to
gdb.texinfo, but got inadvertently dropped when the python related
documentation was split out to python.texi.

gdb/doc/ChangeLog:

	* python.texi (python command): Revise example to match
	command behavior.