DOC: Update to docstring of pd.DataFrame(dtype)
 - [ ] closes #14764 
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

All suggestions to improve this are welcome.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14868?src=pr) is 85.26% (diff: 100%)
> Merging [#14868](https://codecov.io/gh/pandas-dev/pandas/pull/14868?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

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> Powered by [Codecov](https://codecov.io?src=pr). Last update [14e4815...6ee14e7](https://codecov.io/gh/pandas-dev/pandas/compare/14e4815391dcd8c9fe91479fed629410bf63ca33...6ee14e781b3058d36a25dd6d9ca2c29039a2d6c6?src=pr) Personally, I am not convinced that these examples are needed in the DataFrame docstring

- "Leaving dtype=None in constructor will infer a wider type than necessary" -> these are just the default dtypes, so I would phrase it in any case differently
- "A list/dict/Series/array-like is not allowed." -> We should just clarify in the explanation of the parameter above that "only a single dtype is allowed". Or maybe rephrase the example to just show that you can specify a single dtype.
- "Fix up dtypes after declaration" -> Also, I don't think this is needed here. Could maybe go somewhere in the tutorial docs.
 @patniharshit Can you update? yeah sure can you rebase / update I am extremely sorry to say that I won't be able to complete this now. Either someone take this up or feel free to close/delete this. thanks @patniharshit np problem.