Improve 3D switch block commenting and move command line option to arch file

[vaughnbetz]

PR #2370 has some suggested commenting updates; please take a look and integrate them as we merged that PR so it wasn't hanging around too long.

Proposed Behaviour

• Move --custom_3d_sb_fanin_fanout to the arch file instead. Not sure what section is best ... maybe in the custom switch block or the switch block list is the most logical.
• Add a big comment somewhere saying how custom_3d_sb_fanin_fanout works. It should detail exactly what the rr graph this generates looks like.
• Add a comment describing why you are checking if you found nodes on the target layer in the high fanout route expansion https://github.com/verilog-to-routing/vtr-verilog-to-routing/pull/2370/files/b681f8d4afc16ab837c5a03cbfa353a17098be37#diff-94aa757a7f9f207973239bdae6ecbbdd7a8c7e916d4d6f3ce3a349adc849d6c5R1022