Add documentation for NodeType and EdgeType (#1043)

This commit adds some docstrings for the concepts of NodeType and
EdgeType. I also swapped the order so that `NodeType` comes first,
which is more consistent with usage across the codebase.

This commit makes no changes to the actual code; the only effects
are re-organization and documentation.

Test plan: `yarn test` && human inspection

src/analysis/types.js

@@ -2,17 +2,95 @@
 
 import {type NodeAddressT, type EdgeAddressT} from "../core/graph";
 
-export type EdgeType = {|
+/**
-  +forwardName: string,
+ * This module defines `NodeType`s and `EdgeType`s, both of which are
-  +backwardName: string,
+ * data structures containing shared metadata that describes many nodes or edges
-  +defaultForwardWeight: number,
+ * in the graph. Nodes can be "members" of zero or more `NodeType`s, and edges can
-  +defaultBackwardWeight: number,
+ * be "members" of zero or more `EdgeType`s. Membership is determined based on the
-  +prefix: EdgeAddressT,
+ * type's `prefix`, which is an address. A node or edge is considered a member of
-|};
+ * a type if that type's prefix is a prefix of the node's address.
+ *
+ * To make this more concrete, let's consider a specific example. Suppose we define
+ * the following nodes:
+ *
+ * const pullNode = NodeAddress.fromParts(["github", "pull", "repo", "34"]);
+ * const commitNode = NodeAddress.fromParts(["git", "commit", "e1337"]);
+ * const pullType: NodeType = {
+ *  name: "Pull request",
+ *  prefix: NodeAddress.fromParts(["github", "pull"]),
+ *  // ... more properties as required
+ * };
+ * const githubType: NodeType = {
+ *   name: "GitHub node",
+ *   prefix: NodeAddress.fromParts(["github"])
+ * };
+ *
+ * Then the `pullNode` is considered a member of the `pullType` and `githubType`,
+ * while the `commitNode` is not a member of either type.
+ *
+ * The concept of a "type" is useful to SourceCred because it let's us express
+ * that groups of nodes are conceptually related, and that we should treat them similarly.
+ * Most saliently, we use types to assign default weights to groups of nodes, and to
+ * aggregate them for better UI organization. The fact that the SourceCred UI can group
+ * all pull requests together, and assign a default weight to all of them, is possible
+ * because the GitHub plugin defines a pull request node type.
+ *
+ * While a node or edge can theoretically be a member of multiple types, in practice we
+ * generally treat the node or edge as though it is only a member of its most specific type.
+ * In the example above, we would treat any individual pull request as though it is only
+ * a member of the pull request node type. That may change in the future. In general,
+ * the type system is not wholly finalized; when it does become finalized, we will
+ * likely move it into src/core. See [#710] for context.
+ *
+ * [#710]: https://github.com/sourcecred/sourcecred/issues/710
+ */
 
+/**
+ * Represents a "Type" of node in the graph. See the module docstring
+ * for context.
+ */
 export type NodeType = {|
+  // The name for an individual node of this type.
+  // For example, for the GitHub PULL_REQUEST node type, the name is "Pull
+  // request". The first letter of the name should be capitalized.
   +name: string,
+  // The name for multiple nodes of this type.
+  // For example, for the GitHub PULL_REQUEST node type, the pluralName is
+  // "Pull requests". The first letter of this name should be capitalized.
   +pluralName: string,
+  // The address that will be used to test whether a node is a member
+  // of this NodeType. A given node `n` is a member of the type `t` if
+  // `NodeAddress.hasPrefix(n, t.prefix) == true`
   +prefix: NodeAddressT,
+  // The default weight to assign to nodes of this type. We use `1` to mean "of
+  // normal importance", and the weights scale linearly from there (i.e. 2
+  // means twice as important).
   +defaultWeight: number,
 |};
+
+/**
+ * Represents a "Type" of edge in the graph. See the module docstring
+ * for context.
+ */
+export type EdgeType = {|
+  // A brief descriptive name of what the "forward" direction of the edge
+  // means. For example, for the GitHub REFERENCES edge type, the forwardName
+  // is "references"
+  +forwardName: string,
+  // A brief descriptive name of what the "backward" direction of the edge
+  // means. For example, for the GitHub REFERENCES edge type, the backwardName
+  // is "referenced by"
+  +backwardName: string,
+  // The default weight for the forward direction of this edge.
+  // We use `1` as a default value ("of normal importance").
+  // The weights have linear importance, i.e. 2 is twice as important as 1.
+  +defaultForwardWeight: number,
+  // The default weight for the backward direction of this edge.
+  // We use `1` as a default value ("of normal importance").
+  // The weights have linear importance, i.e. 2 is twice as important as 1.
+  +defaultBackwardWeight: number,
+  // The address that will be used to test whether an edge is a member
+  // of this EdgeType. A given edge `e` is a member of the type `t` if
+  // `EdgeAddress.hasPrefix(e.address, t.prefix) == true`
+  +prefix: EdgeAddressT,
+|};