gdb/doc: update docs for 'info win' and 'winheight' commands

gdb/doc: update docs for 'info win' and 'winheight' commands

This started by noticing that the docs for 'winheight' are out of
date, the docs currently give a specific list of possible window
names.  However, now that windows can be implemented in Python, it is
not possible to list all possible names.

I now link the user to a mechanism by which they can discover the
valid names for themselves at run time (by using 'info win').  That,
and the fact that gdb provides tab-completion of the name at the
command line, feels good enough.

Finally, I noticed that the docs for 'win info' don't explicitly say
that the name of the window is given in the output.  This could
probably have been inferred, but given I'm now linking to this as a
mechanism to find the window name, I'd prefer to mention that the name
can be found in the output.