Add secondary PostgreSQL memory requirement to troubleshooting

What does this MR do?

While testing geo setup with GET, we found that secondary site PostgreSQL memory settings need to match the primary site PostgreSQL memory. This MR is to document the error in the Geo Troubleshooting.

Related issues

#381585 (comment 1664003542)

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doc/administration/geo/replication/troubleshooting.md

@@ -682,6 +682,19 @@ In GitLab 13.4, a seed project is added when GitLab is first installed. This mak
 on a new Geo secondary site. There is an [issue to account for seed projects](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5618)
 when checking the database.
 
+### Message: `FATAL:  could not map anonymous shared memory: Cannot allocate memory`
+
+If you see this message, it means that the secondary site's PostgreSQL tries to request memory that is higher than the available memory. There is an [issue](https://gitlab.com/gitlab-org/gitlab/-/issues/381585) that tracks this problem. 
+
+Example error message in Patroni logs (located at `/var/log/gitlab/patroni/current` for Linux package installations):
+
+```plaintext
+2023-11-21_23:55:18.63727 FATAL:  could not map anonymous shared memory: Cannot allocate memory
+2023-11-21_23:55:18.63729 HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 17035526144 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.
+```
+
+The workaround is to increase the memory available to the secondary site's PostgreSQL nodes to match the memory requirements of the primary site's PostgreSQL nodes.
+
 ## Synchronization errors
 
 ### Reverify all uploads (or any SSF data type which is verified)