[Taler] docs: added more info about how date is parsed in request params

[Sebastian Javier Marchano]

I have added more information about how a `date` is parsed:

Time threshold in human readable format (yyyy/MM/dd HH:mm:ss)

here: https://docs.taler.net/core/api-merchant.html#inspecting-orders

I did not find another place where it should be changed.

But looking at the source code this is parsed using GNUNET_STRINGS_fancy_time_to_absolute and this function handles a lot of options, should we point in the description to another place/document?

What do you think about supporting timestamps? 

Current supported formats depend on the locale/timezone of the server and the client may have a different one.

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Sebastian