Document specFromNodeId better

[benjie]

In particular, on these pages:

• https://postgraphile.org/postgraphile/next/node-id/
• https://postgraphile.org/postgraphile/next/make-extend-schema-plugin
• https://postgraphile.org/postgraphile/next/migrating-from-v4/make-extend-schema-plugin/#buildgettypeandidentifiersfromnodeid
• https://grafast.org/grafast/step-library/standard-steps/node#specfromnodeid

Context: https://discord.com/channels/489127045289476126/498852330754801666/1139200279452070008

Also, note that that last link contains pgUpdate command which it shouldn't since it's in the standard step library. Or if it does, it should have more prose explaining why we've used that example.