doc: Rewrite initial text for module section

doc/src/04000-modules.md

@@ -1,20 +1,13 @@
 # Modules {#sec:modules}
 
 A module is a functionality of the program.
+There is a huge list of modules available in the imag core distribution.
 
-A module MAY store data in the store (@sec:thestore).
-It MAY include user input in the data it stores in the store.
-A module MUST HAVE a commandline interface, though a module always consists of
-two parts:
+From a naming perspective, we do not differ between low-level and high-level
+modules. Some of the modules shipped with imag cover core functionality such as
+linking, tagging or references to files outside of the store or even the store
+interface itself (which by the way shouldn't be used by the end-user at all).
+Others cover things like diary, notes, wiki or bookmarks.
 
-- A Library
-- A Binary, which
-    * is executable by the user
-    * implements a commandline frontend to the libray of the module
-
-By definition, the Binary depends on the Library.
-By definition, the Library depends on the libstore (@sec:libstore).
-
-A module MUST use the runtime library to implement the commandline
-interface as defined in @sec:librt.
+The modules try to offer a consistent commandline user interface.