Move package docs to own file for discoverablity.

Per suggestion in https://github.com/ipld/go-ipld-prime/pull/49#discussion_r389606762 .

fluent/doc.go

+/*
+	The fluent package offers helper utilities for using NodeAssembler
+	more tersely by providing an interface that handles all errors for you,
+	and allows use of closures for any recursive assembly
+	so that creating trees of data results in indentation for legibility.
+
+	Note that the fluent package creates wrapper objects in order to provide
+	the API conveniences that it does, and this comes at some cost to performance.
+	If you're optimizing for performance, using the fluent interfaces may be inadvisable.
+	However, as with any performance questions, benchmark before making decisions;
+	its entirely possible that your performance bottlenecks will be elsewhere
+	and there's no reason to deny yourself syntactic sugar if the costs don't
+	detectably affect the bottom line.
+*/
+package fluent

fluent/fluentBuilder.go

-/*
-	The fluent package offers helper utilities for using NodeAssembler
-	more tersely by providing an interface that handles all errors for you,
-	and allows use of closures for any recursive assembly
-	so that creating trees of data results in indentation for legibility.
-
-	Note that the fluent package creates wrapper objects in order to provide
-	the API conveniences that it does, and this comes at some cost to performance.
-	If you're optimizing for performance, using the fluent interfaces may be inadvisable.
-	However, as with any performance questions, benchmark before making decisions;
-	its entirely possible that your performance bottlenecks will be elsewhere
-	and there's no reason to deny yourself syntactic sugar if the costs don't
-	detectably affect the bottom line.
-*/
 package fluent
 
 import (