remove read_excel keyword NotImplemented error, update documentation #11544
Followed up in #14326

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Towards fixing bug #11544, with respect to comments on merged PR #11870.

I've removed the `NotImplemented` error for `parse_dates` and `date_parser` keywords in `read_excel`, seeing that they are implemented (see discussion in #11870). I've added an example of the intended functionality in the documentation

@grahamjeffries Thanks for working on this!

I am only not sure why you changed many of the tests by adding `parse_dates=True`. Why is that needed? 
Besides that, I think it would be good to add a test that actually tests the `parse_dates` keyword behaviour.
 Okay, I'll change the warning so that it is raised only when `parse_dates is True`, add a test, and and update the documentation example.

Re: changes to the tests, in PR #11870 I removed those arguments in the test to avoid raising the `NotImplemented` error which is raised by the `read_excel` modifications in that same PR. Now, I've removed those modifications so I thought I'd add the arguments back. It turns out that it makes no difference to the testing outcomes, so I'm happy to go either way. Please advise which route is preferable.
 For the tests, if it does not make a difference, you don't need to add this back. Rather try to add a test where it _does_ make a difference, so we test the functionality explicitly. 
Eg for the test1.xlsx, the first column is formatted as datetime in excel, not as a string, so indeed in those cases `parse_dates=True` is not needed.
 @grahamjeffries can you update
 can you update according to @jorisvandenbossche comments (and rebase)
 @grahamjeffries Do you have time to update this?
 this would be nice for 0.18.0, but not going to hold us up
 @grahamjeffries can you rebase / update
 @grahamjeffries Do you have some time to update/rebase this?
 status of this?

needs a rebase
 Either this should be merged, or I should fix the docs that use this keyword (now error during doc build), so I would keep it at the 0.19.0 milestone for now to remember that
 can you rebase / update?
 @grahamjeffries can you rebase / update this? 
 I rebased and updated this in https://github.com/pydata/pandas/pull/14326
