improvements for plotattr

I am opening this to get comments and ideas on improvements of plotattr that looks up plot keywords from the console (added here #674). Here are some ideas:

• write proper doc-strings for the exported functions
• make descriptions searchable, e.g. by adding a search keyword to the no-positional-arguments method of plotattr that matches any regular expression or string to descriptions
• identifies 'magic' arguments and how to call them
• add some bonus info for certain attributes... like a more complete descriptions of the core series types, etc. (copied from @tbreloff's comment on the PR)
• make it possible to search 'xlims' and get 'lims' (and other such cases)