Searching channels for literal strings like "C++20" produces non-useful results

This issue has been migrated from [#12609](https://github.com/matrix-org/synapse/issues/12609).

---

I first raised this issue in https://github.com/vector-im/element-web/issues/22010; apparently since the room is not encrypted, the search is done on server side, so I was advised to reopen this issue here.

### Description

Searching channels for literal strings like "C++20" produces non-useful results. This happens at least in the room `#please:matrix.org`.

### Steps to reproduce

Search a channel with lots of history for strings like "C++20" or "C++20 modules". Try all of the following (by entering these in the Element search field):
- C++20
- "C++20"
- C\\+\\+20

### Expected results

At least one of the searches finds messages containing one of the literal strings "c++20" or "C++20".

### Actual results:

I got a ton of unrelated hits; the logic seems to be something like:

- it matches all messages with words starting with c and that contain "20"
- of those, it (I don't know if the server or Element) highlights fully all words that start with c, only the letter c in the middle of other words, and all instances of "20".

Here's an example:


### Version information

I do not understand enough about Matrix to be confident in my answers; I merely use it over an Element web interface provided by an organization (https://chat.hacklab.fi). Its Settings->Help and About page says "Homeserver is https://matrix.hacklab.fi".

I managed to get a sensible looking answer for the Synapse version by running this command:

```
$ curl https://matrix.hacklab.fi/_synapse/admin/v1/server_version
{"server_version":"1.57.0","python_version":"3.9.2"}
```

This is all I know about the environment, but if you need to know something more, I can ask around.