[Documentation improvement] Command to grant Server Admin isn't fully explained

### Description

I'm spinning up a new Matrix ecosystem with Synapse and of course Postgresql, and I have urges to do Admin stuff, such as with the Synapse-Admin tooling.

In this particular deployment it's configured to ONLY allow users to login via LDAP/Active Directory interfacing, so localdb and local user/creds is intentionally disabled for Synapse.

When logging in with said LDAP/AD user it is (so far as I can tell) correctly created in the Synapse system and in-turn database.

As per the documentation here: https://element-hq.github.io/synapse/latest/usage/administration/admin_faq.html?highlight=faq#how-do-i-become-a-server-admin

To grant target user Server Admin (so I can do useful things with Synapse-Admin, for example) I need to run this command (adapted to the correct User and HomeServer):

```sql
UPDATE users SET admin = 1 WHERE name = '@user:domain.com';
```

This change is reflected immediately in the database, but in my testing the Synapse instance is not convinced that user is now a Server Admin until the Synapse instance is restarted (in this case container is restarted).

The documentation on above linked URL does not mention this whatsoever, and I recommend that this (or appropriate verbiage) be added to that page as it took me a while to figure it out.

No, I'm not going to send a PR as I have a lot already on my plate, I'm taking the time to write this bug report, and while I can appreciate this is lower on the priority list, I still think that this is worth improving in said documentation. Additionally I am unaware if there is some way to trigger Synapse to "reload"/"reread" database contents that I just don't know yet, so I'm honestly not sufficiently qualified to write said documentation anyways.

Hope this helps some other $futureHuman.

### Steps to reproduce

- list the steps
- that reproduce the bug
- using hyphens as bullet points

### Homeserver

self-hosted one

### Synapse Version

all

### Installation Method

Docker (matrixdotorg/synapse)

### Database

postgresql 17

### Workers

Single process

### Platform

platform is irrelevant

### Configuration

_No response_

### Relevant log output

```shell
M_FORBIDDEN (403): You are not a server admin
```

### Anything else that would be useful to know?

_No response_

Start with the “How do I become a server admin?” section of the linked Synapse admin FAQ and verify whether a restart or another supported reload is required after the SQL update. Update that documentation with the confirmed behavior and validate the documentation build or checks if available; done means the procedure explains when the new admin status takes effect.

Written by the indexing model from the issue text.