DOC: Clarify behaviour of pd.merge
- [x] Closes #34412 (also attempts to clarify #34273)
- [x] tests added / passed (passes `scripts/validate_docstrings.py`)
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (Not needed)

We should discuss first if this is the expected behavior. Imo that is pretty weird. The join is exactly the same for both cases, but the result is different. Does not seem consistent. @jreback @phofl what should the expected behaviour be? Also, some guidance through this part of the code would help (I believe this is responsible for the behaviour):
https://github.com/pandas-dev/pandas/blob/433c9007781080658553fbef1a4d0c2813b404c0/pandas/core/reshape/merge.py#L876-L884 I think the index from left should be preserved in both cases no? So the second example is incorrect IMO @Bharat123rox can you try to identify the code paths that cause this inconsistency?  Closing for now as I don't think this is something that we should document; probably needs some more discussion on the original issue and a fix to the code instead