2 files changed, 30 insertions, 2 deletions
diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index abdb808438..c142981693 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -20,7 +20,9 @@ recommended for development. More information about WSL can be found at
 <https://docs.microsoft.com/en-us/windows/wsl/install>. Running Synapse natively
 on Windows is not officially supported.
 
-The code of Synapse is written in Python 3. To do pretty much anything, you'll need [a recent version of Python 3](https://wiki.python.org/moin/BeginnersGuide/Download).
+The code of Synapse is written in Python 3. To do pretty much anything, you'll need [a recent version of Python 3](https://www.python.org/downloads/). Your Python also needs support for [virtual environments](https://docs.python.org/3/library/venv.html). This is usually built-in, but some Linux distributions like Debian and Ubuntu split it out into its own package. Running `sudo apt install python3-venv` should be enough.
+
+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`.
 
 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).
 
@@ -169,6 +171,27 @@ To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`:
 SYNAPSE_TEST_LOG_LEVEL=DEBUG trial tests
 ```
 
+By default, tests will use an in-memory SQLite database for test data. For additional
+help with debugging, one can use an on-disk SQLite database file instead, in order to
+review database state during and after running tests. This can be done by setting
+the `SYNAPSE_TEST_PERSIST_SQLITE_DB` environment variable. Doing so will cause the
+database state to be stored in a file named `test.db` under the trial process'
+working directory. Typically, this ends up being `_trial_temp/test.db`. For example:
+
+```sh
+SYNAPSE_TEST_PERSIST_SQLITE_DB=1 trial tests
+```
+
+The database file can then be inspected with:
+
+```sh
+sqlite3 _trial_temp/test.db
+```
+
+Note that the database file is cleared at the beginning of each test run. Thus it 
+will always only contain the data generated by the *last run test*. Though generally
+when debugging, one is only running a single test anyway.
+
 ### Running tests under PostgreSQL
 
 Invoking `trial` as above will use an in-memory SQLite database. This is great for
diff --git a/docs/development/url_previews.md b/docs/development/url_previews.md
index aff3813609..154b9a5e12 100644
--- a/docs/development/url_previews.md
+++ b/docs/development/url_previews.md
@@ -35,7 +35,12 @@ When Synapse is asked to preview a URL it does the following:
    5. If the media is HTML:
       1. Decodes the HTML via the stored file.
       2. Generates an Open Graph response from the HTML.
-      3. If an image exists in the Open Graph response:
+      3. If a JSON oEmbed URL was found in the HTML via autodiscovery:
+         1. Downloads the URL and stores it into a file via the media storage provider
+            and saves the local media metadata.
+         2. Convert the oEmbed response to an Open Graph response.
+         3. Override any Open Graph data from the HTML with data from oEmbed.
+      4. If an image exists in the Open Graph response:
          1. Downloads the URL and stores it into a file via the media storage
             provider and saves the local media metadata.
          2. Generates thumbnails.