Fix a logically incorrect example in the expand() docstring

The docstring was supposed to describe how to disable the log hint, which is
applied before the mul hint, but it instead showed how to disable the mul
hint, which does nothing in this case.
Merge pull request #1772 from asmeurer/expand_doc_fix

Fix a logically incorrect example in the expand() docstring
indexing allows exprs
add matrix block
import matrix block
Block.on_diag
Block assumptions
Block docstring
Block handles single dim inputs
slicing in matrix expressions
test_args for Block
blockcut
slicing with steps
Block -> MatrixSlice
str and latex printing for MatrixSlice
pretty _print_MatrixSlice
handle slices better in MatrixSlice printing
out of bounds checking on slices
blockcut test
MatrixSlice._eval_transpose if symmetric