Document limitation of parsing memory string

During review of https://gerrit.cloudera.org/#/c/13986/, tarmstrong found that ParseUtil::ParseMemSpec() doesn't support parsing strings with "TB" in it. We may want to document this limitation in older version so that users won't specify "TB" in startup flags. Off the top of my head, the scratch space and data cache are probably affected. Any memory limits related flags may also be affected.