Clarify purpose of migration guides (#1916)

### What

Add an editor-only comment to the top of the `_migrating` modules in
`soroban-sdk` and `soroban-token-sdk` explaining that these guides
document the breaking changes between major versions and what developers
must change when upgrading, and are explicitly not a changelog of every
small change.

### Why

The migration guides are the document a developer should read to
understand what they need to change when upgrading the SDK, but nothing
in the files conveyed that intent to people and agents editing them,
leaving the scope ambiguous and inviting unrelated changelog-style
entries. I have had to stop agents modifying the file adding minor
changelog-like details on numerous occassions, and us humans have made
the mistake at least once that I'm aware.

### Known limitations

N/A

---
_Generated by [Claude
Code](https://claude.ai/code/session_01D5RjB5uUPKmfFEP6rVZEWL)_

Author: leighmcculloch

soroban-sdk/src/_migrating.rs

@@ -1,3 +1,9 @@
+// NOTE FOR EDITORS: These migration guides describe the breaking changes
+// between major versions and what a developer needs to change when upgrading
+// the SDK from one major version to another. They are not a changelog: not
+// every small change is captured here. This is the document a developer should
+// read to understand what they need to change when upgrading.
+
 //! # Migrating from v26 to v27
 //!
 //! 1. [`bytes!` and `bytesn!` no longer accept base10 (decimal) integer literals][v27_bytes_literals].

soroban-token-sdk/src/_migrating.rs

@@ -1,3 +1,9 @@
+// NOTE FOR EDITORS: These migration guides describe the breaking changes
+// between major versions and what a developer needs to change when upgrading
+// the SDK from one major version to another. They are not a changelog: not
+// every small change is captured here. This is the document a developer should
+// read to understand what they need to change when upgrading.
+
 //! # Migrating from v23 to v26
 //!
 //! 1. Remove the deprecated event format. For details, see the [migration guide for v23 contract events].