Where can I find a complete documentation for enyim and memcached?
Using memcached: How to scale your website easily seems to be the most complete and definitive guide for using memcached. There are .NET examples in this book and the examples are using the enyim client api. It only comes in eBook format (pdf, epub, mobi), but only costs $9 and it's worth every dollar.
As far as documentation on the enyim website, the only part I found useful was the Discussions section.
For the Enyim.Caching library, I'm just using on Visual Studio 2008's object browser so that I could check the use of every class, method or constant.
I found enyim client documentation on github. Actually taught me a few things... http://wiki.github.com/enyim/EnyimMemcached/mecachedclient-usage
The .Increment() function can return an error (the exact value depends on the language you're using). Without API documentation to explain what might cause specific failures, about all you can do dump your stack and go on a trial & error hunt for the cause.