Relax restriction that text can't contain NUL characters.

doc/encoding.md

@@ -55,10 +55,13 @@ from C++, where the `bool` type takes a whole byte.
 The built-in blob types are encoded as follows:
 
 * `Data`:  Encoded as a pointer, identical to `List(UInt8)`.
-* `Text`:  Like `Data`, but the content must be valid UTF-8, the last byte of the content must be
-  zero, and no other byte of the content can be zero.  Note that the NUL terminator is included in
-  the size sent on the wire, but the runtime library should not count it in any size reported to
-  the application.
+* `Text`:  Like `Data`, but the content must be valid UTF-8, and the last byte of the content must
+  be zero.  The encoding allows bytes other than the last to be zero, but some applications
+  (especially ones written in languages that use NUL-terminated strings) may truncate at the first
+  zero.  If a particular text field is explicitly intended to support zero bytes, it should
+  document this, but otherwise senders should assume that zero bytes are not allowed to be safe.
+  Note that the NUL terminator is included in the size sent on the wire, but the runtime library
+  should not count it in any size reported to the application.
 
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