Update welcome.md

Cleanup call for documentation help.  Trying to make it clear these docs only apply to the current development version of MonoGame only.
Update custom_effects.md

Fixed formatting error.
Update custom_effects.md

Added note about preprocessor.
Merge branch 'docs' of github.com:SickheadGames/MonoGame into docs
Latest dependencies with SharpDoc update.
Merge pull request #2929 from SickheadGames/docs

Documentation Fixes