Add note about incompleteness of CBs

The docs kinda imply that circuit breakers protect against OOMEs, at
least that's how some customers seem to interpret them. This commit adds
a note spelling out that this isn't the case.

Author: DaveCTurner

docs/reference/modules/indices/circuit_breaker.asciidoc

@@ -2,7 +2,16 @@
 === Circuit breaker settings
 [[circuit-breaker-description]]
 // tag::circuit-breaker-description-tag[]
-{es} contains multiple circuit breakers used to prevent operations from causing an OutOfMemoryError. Each breaker specifies a limit for how much memory it can use. Additionally, there is a parent-level breaker that specifies the total amount of memory that can be used across all breakers.
+{es} contains multiple circuit breakers used to prevent operations from using an excessive amount of memory. Each breaker tracks the memory
+used by certain operations and specifies a limit for how much memory it may track. Additionally, there
+is a parent-level breaker that specifies the total amount of memory that may be tracked across all breakers.
+
+When a circuit breaker reaches its limit, {es} will reject further operations. See <<circuit-breaker-errors>> for information about errors
+raised by circuit breakers.
+
+Circuit breakers do not track all memory usage in {es} and therefore provide only incomplete protection against excessive memory usage. If
+{es} uses too much memory then it may suffer from performance issues and nodes may even fail with an `OutOfMemoryError`. See
+<<high-jvm-memory-pressure>> for help with troubleshooting high heap usage.
 
 Except where noted otherwise, these settings can be dynamically updated on a
 live cluster with the <<cluster-update-settings,cluster-update-settings>> API.