Avoid misleading attribute-reference in iiasa.Connection docstring

[danielhuppmann]

Description of PR

Per an observation by @Rlamboll, the current docstring of the iiasa.Connection class is misleading because it states

See pyam.iiasa.Connection.valid_connections for a list of available APIs.

when a user actually has to do pyam.iiasa.Connection().valid_connections.

This PR changes the docstring to only show the attribute-name in the docstring (instead of pyam.iiasa...) while maintaining a functioning link to the docs of that attribute.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f6eb201) 94.7% compared to head (0990ddc) 94.7%.

❗ Current head 0990ddc differs from pull request most recent head 9afc8d0. Consider uploading reports for the commit 9afc8d0 to get more accurate results

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #800   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
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  Files         62      62           
  Lines       5999    5999           
=====================================
  Hits        5683    5683           
  Misses       316     316           

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[Rlamboll]

This works, but read_iiasa and lazy_read_iiasa have basically identical docstrings with the same problem (and you aren't referring to attributes of that class there).

[Rlamboll]

I hope people will understand it better this way! Thanks!