gdb/doc: space out list entries, fix one type

gdb/doc: space out list entries, fix one type

I want to add an item to this list, but it's so packed I have trouble
finding where one item ends and the next starts.  Add a few empty lines
to make it a bit more readable.

Doing this, I also noticed that an "aix-thread" should in fact be
"aix-solib".

gdb/doc/ChangeLog:

	* gdb.texinfo (Debugging Output): Add empty lines, fix typo.

Change-Id: I7ef211f9e3988cfbc6ec94124d23a5f2412f3c82