Specify that email handling converts to lowercase first

Spec for https://github.com/matrix-org/matrix-doc/pull/2265

content/appendices.md

@@ -772,6 +772,13 @@ Represents E-Mail addresses. The `address` is the raw email address in
 `user@domain` form with the domain in lowercase. It must not contain
 other text such as real name, angle brackets or a mailto: prefix.
 
+In additional to lowercasing the domain component of an email address,
+implementations are expected to lowercase the localpart as described
+in [the unicode mapping file](https://www.unicode.org/Public/8.0.0/ucd/CaseFolding.txt)
+prior to any processing. For example, `Strauß@Example.com` must be
+considered to be `strauss@example.com` while processing the email
+address.
+
 ### PSTN Phone numbers
 
 Medium: `msisdn`

data/api/identity/v2_lookup.yaml

@@ -101,6 +101,10 @@ paths:
                   The addresses to look up. The format of the entries here depend on
                   the `algorithm` used. Note that queries which have been incorrectly
                   hashed or formatted will lead to no matches.
+
+                  Note that addresses are case sensitive: review the
+                  [3PID Types](/appendices#pid-types) to verify the intended case an
+                  identifier should be prior to submission/hashing.
                 example: [
                   "4kenr7N9drpCJ4AfalmlGQVsOn3o2RHjkADUpXJWZUc",
                   "nlo35_T5fzSGZzJApqu8lgIudJvmOQtDaHtr-I4rU7I"