Merge pull request #4080 from kquick/kwq/flake-int-doc

Add some internal documentation for flake support objects.

src/libfetchers/fetchers.hh

@@ -21,6 +21,14 @@ struct Tree
 
 struct InputScheme;
 
+/* The Input object is generated by a specific fetcher, based on the
+ * user-supplied input attribute in the flake.nix file, and contains
+ * the information that the specific fetcher needs to perform the
+ * actual fetch.  The Input object is most commonly created via the
+ * "fromURL()" or "fromAttrs()" static functions which are provided
+ * the url or attrset specified in the flake file.
+ */
+
 struct Input
 {
     friend struct InputScheme;
@@ -84,6 +92,16 @@ public:
     std::optional<time_t> getLastModified() const;
 };
 
+
+/* The InputScheme represents a type of fetcher.  Each fetcher
+ * registers with nix at startup time.  When processing an input for a
+ * flake, each scheme is given an opportunity to "recognize" that
+ * input from the url or attributes in the flake file's specification
+ * and return an Input object to represent the input if it is
+ * recognized.  The Input object contains the information the fetcher
+ * needs to actually perform the "fetch()" when called.
+ */
+
 struct InputScheme
 {
     virtual ~InputScheme()