Add release note and update doc regarding ICU (#14712)

Fixes #14704.

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>

3 files changed, 30 insertions, 6 deletions

diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index 342bc1d340..d07790f184 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -24,6 +24,8 @@ The code of Synapse is written in Python 3. To do pretty much anything, you'll n
 
 Synapse can connect to PostgreSQL via the [psycopg2](https://pypi.org/project/psycopg2/) Python library. Building this library from source requires access to PostgreSQL's C header files. On Debian or Ubuntu Linux, these can be installed with `sudo apt install libpq-dev`.
 
+Synapse has an optional, improved user search with better Unicode support. For that you need the development package of `libicu`. On Debian or Ubuntu Linux, this can be installed with `sudo apt install libicu-dev`.
+
 The source code of Synapse is hosted on GitHub. You will also need [a recent version of git](https://github.com/git-guides/install-git).
 
 For some tests, you will need [a recent version of Docker](https://docs.docker.com/get-docker/).
diff --git a/docs/setup/installation.md b/docs/setup/installation.md
index 436041f8a8..85b2ac2580 100644
--- a/docs/setup/installation.md
+++ b/docs/setup/installation.md
@@ -278,7 +278,7 @@ Installing prerequisites on Ubuntu or Debian:
 ```sh
 sudo apt install build-essential python3-dev libffi-dev \
                      python3-pip python3-setuptools sqlite3 \
-                     libssl-dev virtualenv libjpeg-dev libxslt1-dev
+                     libssl-dev virtualenv libjpeg-dev libxslt1-dev libicu-dev
 ```
 
 ##### ArchLinux
@@ -287,7 +287,7 @@ Installing prerequisites on ArchLinux:
 
 ```sh
 sudo pacman -S base-devel python python-pip \
-               python-setuptools python-virtualenv sqlite3
+               python-setuptools python-virtualenv sqlite3 icu
 ```
 
 ##### CentOS/Fedora
@@ -297,7 +297,8 @@ Installing prerequisites on CentOS or Fedora Linux:
 ```sh
 sudo dnf install libtiff-devel libjpeg-devel libzip-devel freetype-devel \
                  libwebp-devel libxml2-devel libxslt-devel libpq-devel \
-                 python3-virtualenv libffi-devel openssl-devel python3-devel
+                 python3-virtualenv libffi-devel openssl-devel python3-devel \
+                 libicu-devel
 sudo dnf groupinstall "Development Tools"
 ```
 
@@ -310,8 +311,12 @@ You may need to install the latest Xcode developer tools:
 xcode-select --install
 ```
 
-On ARM-based Macs you may need to install libjpeg and libpq. 
-You can use Homebrew (https://brew.sh):
+Some extra dependencies may be needed. You can use Homebrew (https://brew.sh) for them.
+
+You may need to install icu, and make the icu binaries and libraries accessible.
+Please follow [the official instructions of PyICU](https://pypi.org/project/PyICU/) to do so.
+
+On ARM-based Macs you may also need to install libjpeg and libpq:
 ```sh
  brew install jpeg libpq
  ```
@@ -332,7 +337,8 @@ Installing prerequisites on openSUSE:
 ```sh
 sudo zypper in -t pattern devel_basis
 sudo zypper in python-pip python-setuptools sqlite3 python-virtualenv \
-               python-devel libffi-devel libopenssl-devel libjpeg62-devel
+               python-devel libffi-devel libopenssl-devel libjpeg62-devel \
+               libicu-devel
 ```
 
 ##### OpenBSD
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 4fe9e4f02e..dcae12ec94 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -88,6 +88,22 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.74.0
+
+## Unicode support in user search
+
+This version introduces optional support for an [improved user search dealing with Unicode characters](https://github.com/matrix-org/synapse/pull/14464).
+
+If you want to take advantage of this feature you need to install PyICU,
+the ICU native dependency and its development headers
+so that PyICU can build since no prebuilt wheels are available.
+
+You can follow [the PyICU documentation](https://pypi.org/project/PyICU/) to do so,
+and then do `pip install matrix-synapse[icu]` for a PyPI install.
+
+Docker images and Debian packages need nothing specific as they already
+include or specify ICU as an explicit dependency.
+
 # Upgrading to v1.73.0
 
 ## Legacy Prometheus metric names have now been removed