`<op>.from_string` functions need docstrings

These functions are user-facing and need docstrings (oops!):
```python
gb.agg.from_string
gb.binary.from_string
gb.indexunary.from_string
gb.monoid.from_string
gb.op.from_string
gb.select.from_string
gb.semiring.from_string
gb.unary.from_string
```
These are what power strings to be passed in place of operators, such as `A.mxm(A, "plus.times")` or `A.mxm(A, "+.*[int32]")`.