Document -Xtgc:allocation

[amicic]

Issue or pull request number: https://github.com/eclipse-openj9/openj9/pull/19070

Overview: -Xtgc:allocation has existed 'forever' but just recently (for 0.46) has been enhanced so it may be more useful for users to find thread specific allocation stats, hence more reasons to document it.

An attempt here: https://github.com/eclipse-openj9/openj9-docs/pull/1295

Release target: Eclipse OpenJ9 0.46

Applies to the following JDK versions: All

Applies to the following platforms: All

[pshipton]

Updated to use the template and fill it in.