Add link to Pulsar env var workaround

There is a section in the Pulsar client authentication reference guide
that warns users of the lack of relaxed binding for authentication
parameter map keys.

This lack of relaxed binding prevents users from setting these auth
parameters directly via env var as the casing is lost in translation.

The commit adds a link in this area of the reference guide to a
workaround in the Spring Pulsar framework reference guide.

See gh-39630

Author: onobc

spring-boot-project/spring-boot-docs/src/docs/asciidoc/messaging/pulsar.adoc

@@ -50,6 +50,9 @@ Spring Boot will not attempt any kind of relaxed binding for these entries.
 
 For example, if you want to configure the issuer url for the `AuthenticationOAuth2` auth plugin you must use `+spring.pulsar.client.authentication.param.issuerUrl+`.
 If you use other forms, such as `issuerurl` or `issuer-url`, the setting will not be applied to the plugin.
+
+This lack of relaxed binding also makes using environment variables for authentication parameters problematic because the case sensitivity is lost during translation.
+If you use environment variables for the parameters then you will need to follow {spring-pulsar-docs}reference/pulsar.html#client-authentication-env-vars[these steps] in the Spring for Apache Pulsar reference documentation for it to work properly.
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