Improve online docs for stack unpack

Author: mpilgrem

doc/unpack_command.md

@@ -11,13 +11,25 @@ the package index (e.g. Hackage) and unpacks each archive.
 
 By default:
 
-*   the download is for the most recent version of the package in the package
-    index. Specify the package name and its version (for example,
-    `acme-missiles-0.1.0.0`) for a particular version of the package.
-    Optionally, a specific revision in the package index can be specified by
-    appending `@rev:<number>` or `@sha256:<sha>`; and
+*   if Stack's `--resolver` option is not specified, the download is for the
+    most recent version of the package in the package index. Specify the package
+    name and its version (for example, `acme-missiles-0.1.0.0`) for a particular
+    version of the package. Optionally, a specific revision in the package index
+    can be specified by appending `@rev:<number>` or `@sha256:<sha>`;
+
+*   if Stack's `--resolver` option is specified and the package's version is not
+    specified, the download is for the version of the package included directly
+    in the specified snapshot; and
 
 *   the package is unpacked into a directory named after the package and its
     version. Pass the option `--to <directory>` to specify the destination
     directory. The directory can be an absolute one or relative to the current
     directory.
+
+!!! note
+
+    Stackage snapshots do not include directly most GHC boot packages (packages
+    that come with GHC and are included in GHC's global package database) but
+    some snapshots may include directly some boot packages. In particular, some
+    snapshots include directly `Win32` (which is a boot package on Windows)
+    while others do not.