Add explicit documentation re null bytes to Path.

There's already explicit sections for empty string and slash cases,
on the basis of how likely those are to provoke questions
(even though the content is essentially to highlight how *not*
exceptional they are); it makes just as much sense to do the same
call out for null bytes.

Original text started in
https://github.com/ipld/go-ipld-prime/pull/47/files#r384127483 .
Co-Authored-By: Peter Rabbitson <ribasushi@leporine.io>

path.go

@@ -49,6 +49,12 @@ import (
 // filesystems); but, as with empty strings, maps which contain such a key
 // certainly exist, and it is important that we be able to regard them!)
 //
+// A string starting, ending, or otherwise containing the NUL (\x00) byte
+// is also a valid PathSegment.  This follows from the rule of "a string is
+// regarded as its constituent eight-bit bytes": an all-zero byte is not exceptional.
+// In golang, this doesn't pose particular difficulty, but note this would be
+// of marked concern for languages which have "C-style nul-terminated strings".
+//
 // For an IPLD Path to be represented as a string, an encoding system
 // including escaping is necessary.  At present, there is not a single
 // canonical specification for such an escaping; we expect to decide one