Please update the "Managed Plugins" page to use gmcs instead of mcs

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Relevant page in the documentation: Unity - Manual: Managed plug-ins

DLLs built using an up-to-date version of mcs won’t actually load in the Unity3D editor. Instead, you get an error like:

TypeLoadException: Could not load type 'xxxx' from assembly 'xxxx, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.

Because Unity uses mscorlib 2.6 (or something like it), you should use gmcs instead as described here: C# Compiler | Mono

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I am not the only one learning this the hard way: