Add commented out JVM options for G1GC (#33685)

* Add commented out JVM options for G1GC

These options are available now that we will be supporting G1GC for Java 10 and
above. They are also designed so that the CMS options don't have to be commented
out in order for the G1 options to take effect.

* Update wording

distribution/src/config/jvm.options

@@ -37,6 +37,14 @@
 -XX:CMSInitiatingOccupancyFraction=75
 -XX:+UseCMSInitiatingOccupancyOnly
 
+## G1GC Configuration
+# NOTE: G1GC is only supported on JDK version 10 or later.
+# To use G1GC uncomment the lines below.
+# 10-:-XX:-UseConcMarkSweepGC
+# 10-:-XX:-UseCMSInitiatingOccupancyOnly
+# 10-:-XX:+UseG1GC
+# 10-:-XX:InitiatingHeapOccupancyPercent=75
+
 ## optimizations
 
 # pre-touch memory pages used by the JVM during initialization