[DOC] Update installation steps with information about new group jboss-eap7-jdk11

With current release of EAP7.2.1, there has been added a new group named jboss-eap7-jdk11 on RHEL7 and RHEL8 (not on RHEL6 as there is no JDK11 available). Customer can now install EAP on these two supported platforms via two possible commands:

# yum groupinstall jboss-eap7

where JDK8 is brought in as a dependency or

# yum groupinstall jboss-eap7-jdk11

where JDK11 is brought in as a dependency.

In case that there is already some JDK installed on the system then even though other one is brought in as a dependency the one that is selected as default (via alternatives) is utilized for EAP. In this case customer has to check which is used as a default one and update alternatives configuration or select custom java for EAP via JAVA_HOME property as described in the RPM Service Configuration Properties.

As a result, there is possible to remove/alternate warning in 3.3.2. Installing JBoss EAP (RPM Installation) section.

Summing this up:

• there is required to document newly introduced yum group jboss-eap7-jdk11
• update/remove existing warning in 3.3.2. Installing JBoss EAP (RPM Installation) section