Added param description to remove doxygen warning

Added documentation to the Subdiv2D documentation, based on the previous
C API.

In addition to the Issue template which is far more important i would like to add this so issues will get closed after merging related PRs rather than being reliant on the reviewer.
If someone want to add something here feel free.

Since we've got a new feature here on github i would like to use it to get all relevant information into the reported issues.
https://help.github.com/articles/creating-an-issue-template-for-your-repository/

I started a file but i would like to hear some other options about what to add.

add "steps to reproduce" if no code example is applicable

change to comments from alalek