python doc: Rework Breakpoint.__init__ doc

python doc: Rework Breakpoint.__init__ doc

I find the documentation of the gdb.Breakpoint constructor hard to read
and not very informative, especially since we have added the new
linespec parameters.  There are multiple problems (some are subjective):

- It's not clear that you should use either the spec string or the
  explicit arguments, not both.
- It's not clear what combination of parameters you can use.
- The big block of text describing the arguments is hard to read.
- Currently, it seems like the "spec" argument is mandatory, even though
  it is not (if you use explicit linespec).
- The square bracket nesting

    [arg1 [, arg2[, arg3]]]

  makes it seems like if you specify arg3, you must specify arg1 and
  arg2 (it's not the case here).

This patch tries to address these problems.

gdb/doc/ChangeLog:

	* python.texi (Manipulating breakpoints using Python): Split doc
	of Breakpoint.__init__ in two, split text in multiple
	paragraphs, don't nest parameter square brackets.