[docs] Fix `IterationSummary` section. Add missing `IterationCallback`

Change-Id: I1d10201cd927003fd3d46728d13231d3fbc27f96

docs/source/nnls_solving.rst

diff --git a/docs/source/nnls_solving.rst b/docs/source/nnls_solving.rst
index b3d0091..5ee0b03 100644
--- a/docs/source/nnls_solving.rst
+++ b/docs/source/nnls_solving.rst
@@ -1786,7 +1786,7 @@
 
    Number of groups with one or more elements.
 
-:class:`IterationCallback`
+:class:`IterationSummary`
 ==========================
 
 .. class:: IterationSummary
@@ -1906,6 +1906,8 @@
 
    Time (in seconds) since the user called Solve().
 
+:class:`IterationCallback`
+==========================
 
 .. class:: IterationCallback