Document symbolic backends.

doc/misc.rst

@@ -19,6 +19,19 @@ and say::
 
 In addition, you need to have :mod:`numpy` installed.
 
+Symbolic backends
+=================
+
+:mod:`sumpy` supports two symbolic backends: sympy and SymEngine. To use the
+SymEngine backend, ensure that the `SymEngine library
+<https://github.com/symengine/symengine>`_ and the `SymEngine Python bindings
+<https://github.com/symengine/symengine.py>`_ are installed.
+
+By default, :mod:`sumpy` prefers using SymEngine but falls back to sympy if it
+detects that SymEngine is not installed. To force the use of a particular
+backend, set the environment variable `SUMPY_FORCE_SYMBOLIC_BACKEND` to
+`symengine` or `sympy`.
+
 User-visible Changes
 ====================