Doc: Puma: Do not imply single mode has data integrity concerns

Although there are limitations and potentially stability and performance
concerns running puma in single mode has no data integrity problems as
far as is known. The current message raises concern with the way it is
phrased, so change the phrasing to avoid this implication.

See also the originating discussion for details:
https://gitlab.com/groups/gitlab-org/-/epics/5303#note_1016292201

doc/administration/operations/puma.md

@@ -111,7 +111,7 @@ To change the worker timeout to 600 seconds:
 WARNING:
 This is an experimental [Alpha feature](../../policy/alpha-beta-support.md#alpha-features) and subject to change without notice. The feature
 is not ready for production use. If you want to use this feature, we recommend testing
-with non-production data first. See the [known issues](#puma-single-mode-known-issues)
+outside of production first. See the [known issues](#puma-single-mode-known-issues)
 for additional details.
 
 In a memory-constrained environment with less than 4GB of RAM available, consider disabling Puma
@@ -137,6 +137,10 @@ For details on Puma worker and thread settings, see the [Puma requirements](../.
 The downside of running Puma in this configuration is the reduced throughput, which can be
 considered a fair tradeoff in a memory-constrained environment.
 
+Remember to have sufficient swap available to avoid out of memory (OOM)
+conditions. View the [Memory requirements](../../install/requirements.md#memory)
+for details.
+
 ### Puma single mode known issues
 
 When running Puma in single mode, some features are not supported:

doc/administration/reference_architectures/1k_users.md

@@ -93,7 +93,8 @@ swap on your server, even if you currently have enough available memory. Having
 swap helps to reduce the chance of errors occurring if your available memory
 changes. We also recommend configuring the kernel's swappiness setting to a
 lower value (such as `10`) to make the most of your memory, while still having
-the swap available when needed.
+the swap available when needed. View the
+[Memory requirements](../../install/requirements.md#memory) for details.
 
 ## Setup instructions
 

doc/install/requirements.md

@@ -94,6 +94,14 @@ if your available memory changes. We also recommend configuring the kernel's swa
 to a low value like `10` to make the most of your RAM while still having the swap
 available when needed.
 
+NOTE:
+Although excessive swapping is undesired and degrades performance, it is an
+extremely important last resort against out-of-memory conditions. During
+unexpected system load, such as OS updates or other services on the same host,
+peak memory load spikes could be much higher than average. Having plenty of swap
+helps avoid the Linux OOM killer unsafely terminating a potentially critical
+process, such as PostgreSQL, which can have disastrous consequences.
+
 ## Database
 
 PostgreSQL is the only supported database, which is bundled with the Omnibus GitLab package.