gdb: update Type.fields doc based on actual GDB behavior

I noticed two errors in the Type.fields documentation:

1. It is possible to call `fields` on an array type, in which case it
   returns one field representing the array's range.  It is not
   mentioned.

2. When calling `fields` on a type that doesn't have fields (by nature,
   like an int), GDB raises a TypeError.  It does not return an empty
   sequence, as currently documented.

Fix these, and change the text into a bullet list.  I find it easier to
read than one big paragraph.

The first issue is already tested in gdb.python/py-type.exp, but the
second one doesn't seem tested.  Add a test in gdb.python/py-type.exp
for it.

gdb/doc/ChangeLog:

	* python.texi (Types In Python): Re-organize Type.fields doc.
	Mention handling of array types.  Correct doc for when calling
	the method on another type.

gdb/testsuite/ChangeLog:

	* gdb.python/py-type.exp (test_fields): Test calling fields on
	an int type.

Change-Id: I11c688177504cb070b81a4446ac91dec50b56a22