Document the use of the migration script (#386)

* Document the use of the migration script

* Changelog

* Update docs/casefold_migration.md

* Apply suggestions from code review

Co-authored-by: reivilibre <38398653+reivilibre@users.noreply.github.com>

* Update docs/casefold_migration.md

Co-authored-by: reivilibre <38398653+reivilibre@users.noreply.github.com>

Co-authored-by: reivilibre <38398653+reivilibre@users.noreply.github.com>

changelog.d/386.misc

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+Case-fold email addresses when binding to MXIDs or performing look-ups. Contributed by H. Shay.

docs/casefold_migration.md

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+# Migrating to case-insensitive email addresses
+
+**Note: the operation described in this documentation is only needed if your server was
+running a version of Sydent earlier than 2.4.0 at some point, and only needs to be run
+once. If the first version of Sydent you have set up is 2.4.0 or later, or if you have
+already run this operation, you don't need to do it again.**
+
+In the past, the Matrix specification would consider email addresses as case-sensitive. This means
+`alice@example.com` and `Alice@example.com` would be seen as two different email addresses
+which could each be associated with a different Matrix user ID.
+
+With [MSC2265](https://github.com/matrix-org/matrix-doc/pull/2265), the Matrix
+specification was updated so that email addresses are considered without any case sensitivity (so the two
+addresses mentioned in the previous paragraph would be considered as being one and the
+same).
+
+As of version 2.4.0, Sydent supports this change by processing each new association
+without case sensitivity. However, some data might remain in the database from earlier
+versions when Sydent would support multiple associations for a given email address (by
+using variations of the same address with a different case). This means some addresses in
+your identity server's database might not have been stored in a format that allows for
+case-insensitive processing, or might have duplicate associations.
+
+To correct this, Sydent 2.4.0 introduces a [script](https://github.com/matrix-org/sydent/blob/main/scripts/casefold_db.py)
+that inspects an identity server's database and fixes it to be compatible with this change:
+
+```
+Usage: /path/to/sydent/scripts/casefold_db.py --config_path=/path/to/sydent.conf [--no-email] [--dry-run]
+
+Arguments:
+    * --config_path: path to Sydent's sydent.conf config file
+    * --no-email: don't send out emails when deleting associations due to duplicates
+    * --dry-run: don't update database rows and don't send out emails
+```
+
+If the script finds a duplicate (i.e. an email address with multiple associations), it
+keeps the most recent association and deletes the others. If one or more of the Matrix
+user IDs that are being dissociated don't match the one being kept, the script also sends an
+email to the address to inform the user of the dissocation.
+
+The default template for this email can be found [here](https://github.com/matrix-org/sydent/blob/main/res/matrix-org/migration_template.eml.j2)
+and can be overriden by configuring a custom template directory (by changing the
+`templates.path` configuration setting). The custom template must be named `migration_template.eml.j2`
+(or `migration_template.eml` if not using Jinja 2 syntax), and will be given the Matrix
+user ID being dissociated at render through the variable `mxid`.