Explain main-is error for autogenerated file

Author: philderbeast

doc/cabal-package-description-file.rst

@@ -3026,14 +3026,17 @@ appear twice for a component: once to flag them as being autogenerated in an
 with a listing in :pkg-field:`other-modules` or
 :pkg-field:`library:exposed-modules`.
 
-.. warning::
+.. note::
 
-    ``cabal sdist`` will fail to generate a source distribution reporting that
-    module files are not found if autogenerated files are not listed in
-    :pkg-field:`autogen-modules` fields.
+    Listing modules in :pkg-field:`autogen-modules` exempts ``cabal sdist`` from
+    finding their source files. Creating a source distribution with ``cabal
+    sdist`` reports any source files not found and fails.
 
-    :pkg-field:`executable:main-is` modules are not supported on
-    :pkg-field:`autogen-modules`.
+.. error::
+
+    An autogenerated module cannot also be the :pkg-field:`executable:main-is`
+    file. The ``cabal sdist`` command expects to find this file in the source
+    tree and autogenerated modules are generated outside the source tree.
 
 .. pkg-field:: autogen-modules: module list
    :since: 2.0