added header to the codebook table

Author: DerAndereJohannes

docs/codebook.rst

@@ -1,7 +1,7 @@
 Codebook
 ========
 
-Here you can download the complete codebook which details the structure of data used throughout this documentation.
+Here you can download the complete codebook which details the variables that you can compute using the NeuroKit package.
 
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@@ -11,7 +11,11 @@ Here you can download the complete codebook which details the structure of data
        </a>
    </div>
 
-This codebook contains detailed descriptions of all variables, their possible values, and additional metadata.
+This codebook contains detailed descriptions of all variables, their descriptions, and additional metadata.
+
+
+Codebook Table
+==============
 
 .. raw:: html