Add Tom's comments to the docs

I find @tomprince's comments valuable for understanding what is supposed to live where.

Source: https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/merge_requests/237#note_18712

morph/README.rst

@@ -36,11 +36,17 @@ lib
 ---
 
 This contains Nix library code for defining the grids.
+It has all the details of how each type of node in our grid is configured.
+It knows about morph (so defines ``deployment.secrets`` and has the logic for collecting data defined by other nodes).
+It defines options (i.e. ``grid.*``) for things specific to how we configure grids (e.g. ``grid.publicKeyPath``).
+It defines metadata about nodes that we use on other nodes (e.g. ``grid.monitoringvpnIPv4`` which is used to define various things on the monitoring node).
+Each top-level module here defines one type of node with all (or at least most) of the configuration necessary for that node.
 
 grid
 ----
 
 Specific grid definitions live in subdirectories beneath this directory.
+They consist almost exclusively setting options defined in ``morph/lib`` (and few options defined elsewhere) and then delegating to the ``morph/lib`` modules.
 
 private-keys
 ~~~~~~~~~~~~