analyzer: fix and clarify region model example in internals docs

The Region Model example in the analyzer internals docs had a few
inconsistencies with the dumps. Fix these, and add a note that
heap-allocated region numbering depends on traversal order.

gcc/ChangeLog:

	* doc/analyzer.texi (Region Model): Use malloc (n) in the
	example to match the dump; fix the frame for the result_4
	cluster; correct the heap region number; note that heap region
	numbering depends on traversal order.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@gmail.com>