doc: mention the pointer size constraint

We check in a variety of places that our pointer files are less than 1
KiB in size.  Because this is a limitation, let's document it in the
spec so that users can easily understand the constraint.

Author: bk2204

docs/spec.md

@@ -19,6 +19,7 @@ only a pointer file is written.
 * Values MUST NOT contain return or newline characters.
 * Pointer files MUST be stored in Git with their executable bit matching that
 of the replaced file.
+* Pointer files must be less than 1024 bytes in size.
 * Pointer files are unique: that is, there is exactly one valid encoding for a
   pointer file.