Autogen: Doc: Add Visual Studio PRE_BUILD note to cmake-qt.rst

Help/manual/cmake-qt.7.rst

@@ -214,6 +214,24 @@ overrides options from the :prop_tgt:`AUTORCC_OPTIONS` target property.
 Source files can be excluded from :prop_tgt:`AUTORCC` processing by
 enabling :prop_sf:`SKIP_AUTORCC` or the broader :prop_sf:`SKIP_AUTOGEN`.
 
+Visual Studio Generators
+========================
+
+When using the :manual:`Visual Studio generators <cmake-generators(7)>`
+CMake tries to use a ``PRE_BUILD``
+:command:`custom command <add_custom_command>` instead
+of a :command:`custom target <add_custom_target>` for autogen.
+``PRE_BUILD`` can't be used when the autogen target depends on files.
+This happens when
+
+- :prop_tgt:`AUTOMOC` or :prop_tgt:`AUTOUIC` is enabled and the origin target
+  depends on :prop_sf:`GENERATED` files which aren't excluded from autogen by
+  :prop_sf:`SKIP_AUTOMOC`, :prop_sf:`SKIP_AUTOUIC`, :prop_sf:`SKIP_AUTOGEN`
+  or :policy:`CMP0071`
+- :prop_tgt:`AUTORCC` is enabled and a ``.qrc`` file is listed in
+  the origin target sources
+- :prop_tgt:`AUTOGEN_TARGET_DEPENDS` lists a source file
+
 qtmain.lib on Windows
 =====================