Fix various typos in docs (#17114)

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
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The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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* [X] [Code
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(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

docs/message_retention_policies.md

@@ -51,8 +51,8 @@ clients.
 
 ## Server configuration
 
-Support for this feature can be enabled and configured by adding a the
-`retention` in the Synapse configuration file (see
+Support for this feature can be enabled and configured by adding the
+`retention` option in the Synapse configuration file (see
 [configuration manual](usage/configuration/config_documentation.md#retention)).
 
 To enable support for message retention policies, set the setting
@@ -117,7 +117,7 @@ In this example, we define three jobs:
   policy's `max_lifetime` is greater than a week.
 
 Note that this example is tailored to show different configurations and
-features slightly more jobs than it's probably necessary (in practice, a
+features slightly more jobs than is probably necessary (in practice, a
 server admin would probably consider it better to replace the two last
 jobs with one that runs once a day and handles rooms which
 policy's `max_lifetime` is greater than 3 days).