csharp: Fix build and doc generation re beta stuff

csharp: Fix build and doc generation re beta stuff

Summary:
When skipping beta items, make sure generated documentation does not try
to refer to beta stuff using `see` tags. Doing so would lead to warnings
when generating the xml doc file and as we now treat warnings as errors,
compilation would fail.

Instead of `see` tags, use a simple `span` with a suffix text explaining
said item is beta.

Reviewers: vitor.sousa, felipealmeida, segfaultxavi

Reviewed By: segfaultxavi

Subscribers: cedric, #reviewers, #committers

Tags: #efl, #expertise_solutions

Differential Revision: https://phab.enlightenment.org/D9258