Stop advertising broken datalad -c :<key> to unset config

[mih]

datalad/datalad-next#677 highlights some problems from the perspective of implementations in datalad-next.

However, this feature is broken and broken in a way that cannot be addressed cleanly and comprehensively, because Git itself does not support temporary unsetting of config items (only temporary overrides).

The current promise is:

  -c (:name|name=value)
     specify configuration setting overrides. They override any
     configuration read from a file. A configuration can also be unset
     temporarily by prefixing its name with a colon (':'), e.g.
     ':user.name'. Overrides specified here may be overridden themselves
     by configuration settings declared as environment variables.

But the correct description is more like:

A configuration can also be unset temporarily by prefixing its name
with a colon (':'), e.g. ':user.name'. However, this is only in effect
for configuration queries via DataLad's `ConfigManager`, and only in the
root process. Therefore this approach will generally not be effective
for impacting the behavior of Git, git-annex, or DataLad subprocesses
(special remotes).  

I suggest to adjust the documentation now and also deprecate this feature because of the limited scope and large potential to cause confusion and undesired behavior.

[christian-monch]

That is a good suggestion from my point of view