Update docs in observable_estimation

[kylegulshen]

Description

Some of the docs had outdated explanations about compilation and reference to _measure_bitstrings which was moved with the symmetrization code. I highlighted some important cautionary notes with the CAUTION domain.

Checklist

• [x] The above description motivates these changes.
• [ ] There is a unit test that covers these changes.
• [ ] The code respects the API separation discussed in .github/CONTRIBUTING.md.
• [ ] Relevant references and equations are cited using our standard reference style.
• [ ] All new and existing tests pass locally and on Semaphore.
• [ ] Parameters have type hints with PEP 484 syntax.
• [x] Functions and classes have useful sphinx-style docstrings.
• [ ] (New Feature) The docs and example notebooks have been updated accordingly.
• [ ] (Bugfix) The associated issue is referenced above using auto-close keywords.
• [ ] The changelog (docs/source/changes.rst) has a description of this change.

[jlapeyre]

It looks like the commits in this PR were included in #199, which has already been merged. So this PR (#198) is redundant, and may be closed.