Document lazy dependency in init build.zig.zon

Also added the same description to doc/build.zig.zon.md

Author: TUSF

doc/build.zig.zon.md

@@ -63,6 +63,13 @@ When this is provided, the package is found in a directory relative to the
 build root. In this case the package's hash is irrelevant and therefore not
 computed. This field and `url` are mutually exclusive.
 
+#### `lazy`
+
+Boolean.
+
+When this is set to `true`, a package is declared to be lazily fetched. This
+makes the dependency only get fetched if it is actually used.
+
 ### `paths`
 
 List. Required.

lib/init/build.zig.zon

@@ -37,6 +37,11 @@
         //    // build root. In this case the package's hash is irrelevant and therefore not
         //    // computed. This field and `url` are mutually exclusive.
         //    .path = "foo",
+
+        //    // When this is set to `true`, a package is declared to be lazily
+        //    // fetched. This makes the dependency only get fetched if it is
+        //    // actually used.
+        //    .lazy = false,
         //},
     },