Document --max-running-jobs option of gnt-cluster modify
Document --max-running-jobs option of gnt-cluster modify

This option control the number of jobs in non-finalized non-queued
state and was added some time ago. Document it in the man page as
well.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

leshchevds/ganeti,bitemyapp/ganeti,apyrgio/ganeti,grnet/snf-ganeti,dimara/ganeti,andir/ganeti,andir/ganeti,ganeti/ganeti,ganeti/ganeti,yiannist/ganeti,yiannist/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,mbakke/ganeti,onponomarev/ganeti,dimara/ganeti,onponomarev/ganeti,apyrgio/ganeti,grnet/snf-ganeti,andir/ganeti,mbakke/ganeti