specs/config-files: say that drop-ins are not always required

Also drop the sentence that "drop-ins are not full replacements" — in
practice they are and I'm not sure what this sentence was trying to
achieve.

Resolves #48.

specs/configuration_files_specification.md

@@ -55,11 +55,13 @@ For example, an empty `/etc/foo/bar.conf` means that `/usr/lib/foo/bar.conf` is
 masked and thus not parsed.
 
 ## Drop-ins
-All configuration paths must support drop-ins. Supporting drop-ins means that in
-addition to parsing a full configuration file, an implementation also must parse all
-files in the drop-in directory associated with it, and merge the result in lexicographic
-order. Drop-ins are not full replacements, but must allow specifying a subset of settings
-present in the main configuration files.
+All configuration paths must support drop-ins.
+Supporting drop-ins means that in addition to parsing a full configuration file,
+an implementation also must parse all files in the drop-in directory associated with it,
+and merge the result in lexicographic order.
+This requirement does not apply to configuration file formats where
+automatic combining of multiple files is not feasible,
+for example scripts or structured documents.
 
 For example, in addition to parsing `/etc/foo/bar.conf`, `/etc/foo/bar.conf.d/a.conf` and
 `/etc/foo/bar.conf.d/b.conf` must also be parsed, in this order.