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
Added WindowsGL target to platform target in Content Pipeline project
When MGCB is launched with the "LaunchDebugger" flag exceptions thrown within an importer/processor are allowed to not handled.
Clear output window when a project is closed.
Merge pull request #2934 from SickheadGames/PipelineToolMinorFix

Clear output window when a project is closed.
Merge pull request #2932 from SickheadGames/BreakWithinProcessor

'Debug Mode' Improvement
Updating the ContentManager.cs list as well
Merge pull request #2931 from AndrewCarvalho/develop

Added WindowsGL target to platform target in Content Pipeline project
Split TextureCollection into partial classes
for different graphics APIs.
Add TextureCollection partial classes for PSM and Web to definition file.
Merge branch 'texture-collection' of https://github.com/KonajuGames/MonoGame into texture-collection

Conflicts:
	Build/Projects/MonoGame.Framework.definition