[DPE-5659] Update COS alert rules

[a-velasco]

Issue

Alert rule descriptions do not provide much guidance about what actions the user can take.

Solution

Improve the descriptions of the COS alert rule expressions.

The format should be:

1. What happened, or what is the problem
2. What can the user do to recover, or investigate further (this is the part that's usually missing)

Here is a good example:

summary: PostgresSQL SSL compression active (instance {{ $labels.instance }})
description: |
Database connections have SSL compression enabled. This may add significant jitter in replication delay.
Replicas can turn off SSL compression via `sslcompression=0` in `recovery.conf`.

[a-velasco]

LGTM. Have we tested multi-line descriptions being parsed correctly by AlertManager?

@shayancanonical We have not! Would you be able to try this out? Last time we checked together, the newlines were not rendered correctly. I would be curious to see if they work with this alternative yaml formatting.