Add note about how Stack checks packages for sdist/upload

Author: mpilgrem

doc/commands/sdist_command.md

@@ -39,3 +39,9 @@ By default:
     For futher information, see the
     [`pvp-bounds`](../configure/yaml/non-project.md#pvp-bounds) non-project
     specific configuration option documentation.
+
+!!! note
+
+    Stack checks a package for common mistakes using checks provided by Cabal
+    (the library). The version of Cabal used by Stack to check may be
+    older than the version used by Stack to build.

doc/commands/upload_command.md

@@ -43,8 +43,9 @@ uploads the package to Hackage.
 
 By default:
 
-* the command will check each package for common mistakes. Pass the flag
-  `--ignore-check` to disable such checks;
+* the command will check each package for common mistakes. For further
+  information, see the [`stack sdist` command](sdist_command.md) documentation.
+  Pass the flag `--ignore-check` to disable such checks; and
 
 * Stack will not test the resulting package archive. Pass the flag
   `--test-tarball` to cause Stack to test each resulting package archive, by