add some docu about the project loading sequence

Change-Id: Icb6bb58247724aeb6b9433a8d032718f1ba4babe
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/default_post.prf

@@ -1,3 +1,5 @@
+# This file is loaded by qmake right after loading the actual project file.
+
 contains(TEMPLATE, ".*(lib|app)"):CONFIG += have_target
 
 !have_target:!force_qt: CONFIG -= qt

mkspecs/features/default_pre.prf

@@ -1,3 +1,7 @@
+# This file is loaded by qmake right before each actual project file.
+# Note that evaluating variable assignments from the command line
+# still happens in between these two steps.
+
 load(exclusive_builds)
 CONFIG = \
     lex yacc warn_on debug exceptions depend_includepath \

mkspecs/features/device_config.prf

@@ -1,3 +1,5 @@
+# This file is loaded by some qmakespecs to get early configuration data.
+
 # Load generated qdevice.pri
 DEVICE_PRI = $$[QT_HOST_DATA/get]/mkspecs/qdevice.pri
 exists($$DEVICE_PRI):include($$DEVICE_PRI)

mkspecs/features/qt_config.prf

@@ -1,3 +1,5 @@
+# This file is loaded as one of the last things by all qmakespecs.
+
 QMAKE_QT_CONFIG = $$[QT_HOST_DATA/get]/mkspecs/qconfig.pri
 !exists($$QMAKE_QT_CONFIG)|!include($$QMAKE_QT_CONFIG, "", true) {
    debug(1, "Cannot load qconfig.pri!")

mkspecs/features/spec_post.prf

@@ -1,3 +1,9 @@
+# This file is loaded by qmake right after loading the qmakespec.
+# Afterwards, the project's .qmake.conf and .qmake.cache are loaded
+# (if present).
+# Note that up to this point, nothing specific to a particular SUBDIRS
+# project or build pass can be done.
+
 isEmpty(MAKEFILE_GENERATOR):error("Qmake spec does not set MAKEFILE_GENERATOR.")
 isEmpty(QMAKE_PLATFORM) {
     isEmpty(TARGET_PLATFORM) {

mkspecs/features/spec_pre.prf

@@ -1,3 +1,7 @@
+# This file is loaded by qmake right before loading the qmakespec.
+# At this point, the built-in variables have been set up and the project's
+# .qmake.super was read (if present).
+
 QMAKE_DIR_SEP           = $$DIR_SEPARATOR
 QMAKE_DIRLIST_SEP       = $$DIRLIST_SEPARATOR