doc: Document references.

Suggested by "pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>.
Fixes <https://bugs.gnu.org/34574>.

* doc/guix.texi (Derivations): Document references.

doc/guix.texi

@@ -6238,8 +6238,11 @@ The outputs of the derivation---derivations produce at least one file or
 directory in the store, but may produce more.
 @item
-The inputs of the derivations, which may be other derivations or plain
-files in the store (patches, build scripts, etc.)
+@cindex build-time dependencies
+@cindex dependencies, build-time
+The inputs of the derivations---i.e., its build-time dependencies---which may
+be other derivations or plain files in the store (patches, build scripts,
+etc.)
 @item
 The system type targeted by the derivation---e.g., @code{x86_64-linux}.
@@ -6270,6 +6273,16 @@ of a fixed-output derivation are independent of its inputs---e.g., a
 source code download produces the same result regardless of the download
 method and tools being used.
+@cindex references
+@cindex run-time dependencies
+@cindex dependencies, run-time
+The outputs of derivations---i.e., the build results---have a set of
+@dfn{references}, as reported by the @code{references} RPC or the
+@command{guix gc --references} command (@pxref{Invoking guix gc}).  References
+are the set of run-time dependencies of the build results.  References are a
+subset of the inputs of the derivation; this subset is automatically computed
+by the build daemon by scanning all the files in the outputs.
+
 The @code{(guix derivations)} module provides a representation of
 derivations as Scheme objects, along with procedures to create and
 otherwise manipulate derivations.  The lowest-level primitive to create