RFA: improve documentation for block iteration

[Eli Zaretskii]

> From: Tom Tromey <tromey@redhat.com>
> Date: Tue, 21 Feb 2012 12:28:11 -0700
> 
> This needs a doc review.

OK, with one correction:

> +A @code{gdb.Block} is iterable.  The iterator returns the symbols
> +@pxref{Symbols In Python} local to the block.

The @pxref needs to be in parentheses.

Thanks.