doc: make more clear what METADATA.ini is about
# SpecElektra is a specification language that describes content
# of the global key database.
#
+# Put simply, SpecElektra (configuration in the spec namespace)
+# allows you to describe how the configuration of the system looks
+# like. For example, it allows you to describe which configuration
+# files in which formats are present on the system.
+#
# SpecElektra does not have built-in keywords, instead it is
# completely extensible by metadata interpreted by Elektra's plugins.
#
