Update event_cache_size and global_factor configurations documentation (#17071)

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
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`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/17071.doc

+Update event_cache_size and global_factor configurations documentation.

docs/usage/configuration/config_documentation.md

@@ -1317,6 +1317,12 @@ Options related to caching.
 The number of events to cache in memory. Defaults to 10K. Like other caches,
 this is affected by `caches.global_factor` (see below).
 
+For example, the default is 10K and the global_factor default is 0.5.
+
+Since 10K * 0.5 is 5K then the event cache size will be 5K.
+
+The cache affected by this configuration is named as "*getEvent*".
+
 Note that this option is not part of the `caches` section.
 
 Example configuration:
@@ -1342,6 +1348,8 @@ number of entries that can be stored.
 
   Defaults to 0.5, which will halve the size of all caches.
 
+  Note that changing this value also affects the HTTP connection pool.
+
 * `per_cache_factors`: A dictionary of cache name to cache factor for that individual
    cache. Overrides the global cache factor for a given cache.