many small fixes and documentation improvements,

this should be alpha5.

doc/snippets/MatrixBase_lazy.cpp

@@ -0,0 +1,10 @@
+Matrix2d m; m << 1,2,3,4;
+Matrix2d n;
+n = (m*m).lazy(); // if we did "n = m*m;" then m*m would first be evaluated into
+ // a temporary, because the Product expression has the EvalBeforeAssigningBit.
+ // This temporary would then be copied into n. Introducing this temporary is
+ // useless here and wastes time. Doing "n = (m*m).lazy();" evaluates m*m directly
+ // into n, which is faster. But, beware! This is only correct because m and n
+ // are two distinct matrices. Doing "m = (m*m).lazy();" would not produce the
+ // expected result.
+cout << n << endl;