feat(android): back text messages up, in a build Play does not get

A Messages section sends the SMS added since the last copy, as restorable
<smses> XML, into <folder>/<account>-messages/YYYY. A play flavor has neither
READ_SMS nor the code that reads messages; full is the default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/docs/USERGUIDE.md b/docs/USERGUIDE.md
index 4c5c61e..3047b3f 100644
--- a/docs/USERGUIDE.md
+++ b/docs/USERGUIDE.md
@@ -385,7 +385,7 @@ that matters to you.
 ### Backing up your phone
 
 On the Android application, **Phone → Android Sync** in the side menu backs up
-your phone's photos and contacts to a group.
+your phone's photos, contacts and text messages to a group.
 
 **Where backups go** is chosen once, at the top of the page, for every kind:
 
@@ -430,6 +430,18 @@ your phone's photos and contacts to a group.
 - It runs on any connection, since a copy is small. **Back up now** sends one
   at once if your contacts changed since the last copy.
 
+#### Text messages
+
+- **Turn on** asks for access to your text messages. The first copy holds them
+  all; then, once a day, a new file holds only the ones that arrived or were
+  sent since.
+- The files go by year (`alice-messages/2026/sms-2026-10-10-0900.xml`), in the
+  XML format SMS backup applications can restore onto a phone. Senders are
+  named when contacts backup is also allowed. Multimedia messages (MMS) are
+  not included.
+- The version of the application from the Play Store has no text messages
+  backup: Play allows reading them only to the phone's messaging application.
+
 If a backup cannot go on (the node's disk is full, the folder no longer accepts
 files, somebody else joined the group), it stops, says why once in a
 notification and on the Android Sync page, and tries again the next day.