describe: also describe "configure" targets in more detail

Most built-in rules are just described by their name, assuming they
are known to the user anyway. One exception, however, are "export"
targets, as those serve as interfaces to (logical) repositories and
hence is supposed to have a documentation of the target itself. Over
time, however, "configure" targets have evolved to become internal
interfaces (maybe even for an external target), e.g., to provide
default values and hence make better use of the actual export target.
Therefore it is desirable to provide a bit more information when
asked to describe a "configure" target.
- As "configure" targets have a fixed set of keys, adding a "doc"
  field is a conservative extension; this can be useful to give an
  overview what the target is about.
- An important information of a "configure" target is the target
  that is configured. While this, in general, is an expression, in
  the typical cases, the description is very short (a literal target
  name, or a variable). So we can afford to show the definition.

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/concepts/doc-strings.md b/doc/concepts/doc-strings.md
index ed4ee84b..22115cee 100644
--- a/doc/concepts/doc-strings.md
+++ b/doc/concepts/doc-strings.md
@@ -131,6 +131,16 @@ strings describing the targeted in general and `"config_doc"` is a map
 from (some of) the variables of the `"flexible_config"` to an array of
 strings describing this parameter.
 
+Configure targets
+-----------------
+
+As configure targets often serve as internal interface to external
+export targets (e.g., in order to set a needed configuration), we
+support documentation here as well. As configure targets, being
+built-in, have a fixed set of fields, a `"doc"` field can be used
+for this purpose without conflicts. Again, the `"doc"` field is an
+array of strings describing the target in general.
+
 Presentation of the documentation
 ---------------------------------