DOC: Add note in 1.5.3 about SQLAlchemy warnings
xref #50558 

As discussed in todays dev call, for 1.5.3 the decision was to document that SQLAlchemy warnings can be expected but not addressed.
thx @mroeschke  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 1.5.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -x -m1 eddb6f459a6b32112f614a9118659ef763b50db1
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #50680: DOC: Add note in 1.5.3 about SQLAlchemy warnings'
```

4. Push to a named branch:

```
git push YOURFORK 1.5.x:auto-backport-of-pr-50680-on-1.5.x
```

5. Create a PR against branch 1.5.x, I would have named this PR:

> "Backport PR #50680 on branch 1.5.x (DOC: Add note in 1.5.3 about SQLAlchemy warnings)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @meeseeksdev backport to 1.5.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 1.5.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -x -m1 eddb6f459a6b32112f614a9118659ef763b50db1
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #50680: DOC: Add note in 1.5.3 about SQLAlchemy warnings'
```

4. Push to a named branch:

```
git push YOURFORK 1.5.x:auto-backport-of-pr-50680-on-1.5.x
```

5. Create a PR against branch 1.5.x, I would have named this PR:

> "Backport PR #50680 on branch 1.5.x (DOC: Add note in 1.5.3 about SQLAlchemy warnings)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                