fix: add warning for misleading option (#433)

Author: simonLeary42

library/HTMLPurifier/ConfigSchema/schema/Core.ConvertDocumentToFragment.txt

@@ -7,7 +7,8 @@ This parameter determines whether or not the filter should convert
 input that is a full document with html and body tags to a fragment
 of just the contents of a body tag. This parameter is simply something
 HTML Purifier can do during an edge-case: for most inputs, this
-processing is not necessary.
+processing is not necessary. Warning: Full HTML purification has not
+been implemented. See GitHub issue #7.
 
 --ALIASES--
 Core.AcceptFullDocuments