Add docstrings to Observable class

[jordandsullivan]

Issue Description

The Observable class lacks docstrings for most of its methods. While some might be considered self-explanatory (depending on a person's background), many are not, such as Observable.partition(). Even short @property functions could still benefit from having fully written out explanations of what the property is.

Proposed Solution

Add descriptive docstrings for each method.

[natestemen]

To close this issue we should document the following methods.

1. measure_in
2. The arguments of matrix should be flushed out using the Args: ... pattern.
3. expectation
4. partition
5. from_pauli_string_collections

Documenting these will also ensure they show up in the API-doc!