Add documentation for Problem::IsParameterBlockConstant

https://github.com/ceres-solver/ceres-solver/issues/485

Change-Id: I1d3d7e6bb2a33e2202b71c989164dd805fb54b89

docs/source/nnls_modeling.rst

diff --git a/docs/source/nnls_modeling.rst b/docs/source/nnls_modeling.rst
index 7a86640..c19a4ef 100644
--- a/docs/source/nnls_modeling.rst
+++ b/docs/source/nnls_modeling.rst
@@ -1752,6 +1752,10 @@
 
    Allow the indicated parameter to vary during optimization.
 
+.. function:: bool Problem::IsParameterBlockConstant(const double* values) const
+
+   Returns true if a parameter block is set constant, and false otherwise.
+
 .. function:: void Problem::SetParameterization(double* values, LocalParameterization* local_parameterization)
 
    Set the local parameterization for one of the parameter blocks.