Add clarification: min_mem_size and max_mem_size = NULL/None in h2o.init() uses JVM default

[exalate-issue-sync]

Specify in documentation that when launching an H2O cluster ({{h2o.init()}}), if {{min_mem_size}} and {{max_mem_size}} are not defined, ({{-Xmx}} for command line) then H2O will allocate memory determined by default memory of Java Virtual Machine (JVM). The amount is dependent on the Java version, but generally 25% of the machine's physical memory.

H2O 3 docs - [http://docs.h2o.ai/h2o/latest-stable/h2o-docs/starting-h2o.html|http://docs.h2o.ai/h2o/latest-stable/h2o-docs/starting-h2o.html]
R docs - [http://docs.h2o.ai/h2o/latest-stable/h2o-r/docs/reference/h2o.init.html|http://docs.h2o.ai/h2o/latest-stable/h2o-r/docs/reference/h2o.init.html]
Python docs - [http://docs.h2o.ai/h2o/latest-stable/h2o-py/docs/h2o.html?highlight=init#h2o.init|http://docs.h2o.ai/h2o/latest-stable/h2o-py/docs/h2o.html?highlight=init#h2o.init]

[h2o-ops]

JIRA Issue Migration Info

Jira Issue: PUBDEV-7451
Assignee: hannah.tillman
Reporter: Neema Mashayekhi
State: Resolved
Fix Version: 3.30.0.2
Attachments: N/A
Development PRs: Available

Linked PRs from JIRA

#4527