Add more information to package-manager warnings

Qt Creator automatically inserts the package manager cmake to all projects and in some cases it will result in warnings such as:

/<some_existing_project_folder>/CMakeLists.txt:7:
 warning: Qt Creator: QT_MAINTENANCE_TOOL was not set.  Qt's 
MaintenanceTool
find_package() provider will not be used.  To disable this warning set
QT_CREATOR_SKIP_MAINTENANCE_TOOL_PROVIDER to ON.

Call stack:
  /.../.qtc/package-manager/auto-setup.cmake:379 (qtc_maintenance_provider_missing_variable_warning)
  /.../.qtc/package-manager/auto-setup.cmake:373 (message) 

Since the feature is automatically enabled for everyone while not everyone really needs this feature (or even know about it) the warning may appear confusing. It'd be better to add additional information to the warnings e.g. a link to the documentation of the feature to better inform the user about what "find_package() provider" is and how it affects the users project.